There are two approaches to thinking about the topic for one who cares about distillates expressing their personalities without makeup.
The main one goes: additives are bad.
In most cases, I would agree, though it feels like simplifying the topic a little.
Take Cognac for example: The Cahier des Charges (in French, available here:) defines the use of additives as follows:
"The blending of eaux-de-vie of varying ages and profiles is an inherent practice in the creation of "Cognac". This allows for a consistent production of a product possessing the exact and harmonious organoleptic characteristics sought after.
Only the following methods are permitted:
Color adjustment using E150a caramel (regular caramel),
Sweetening using the products defined in Article 4.9 a) and c) of Regulation 2019-787 to enhance the final taste,
The addition of oak wood chip infusions in hot water (boisé).
Their effect on the obscuration of the eau-de-vie is equal to or less than 4% vol. Obscuration, expressed in % vol, is determined by the difference between the actual alcoholic volume and the gross alcoholic volume. The infusion of wood chips is a traditional method: the type of wood used is consistent with the housing specifications set out in the production standards, and if necessary, the infusion is stabilized by adding eau-de-vie corresponding to the target eau-de-vie."
The addition of oak wood chip infusion is among the traditional methods allowed."
Note: The term "obscuration" in the context of spirits refers to the difference between the actual and apparent alcohol content, which can be affected by the addition of certain substances.
The vast majority of distillate bottles in the appellation have had a combination of some or all of these additives added to them mainly to:
make a young cognac look darker than it naturally is,
make a young cognac taste less harsh (“smoother”),
make a cognac smell and taste like it spent more time aging in a barrel than it actually did.
The aim is to have “consistent products” which the larger brands that make up most cognac sales have based their business on.
Nothing wrong here: it makes sense that a bottle of a worldwide available VS, VSOP, or XO has a somewhat consistent(*) color/taste profile wherever it can be found on the globe.
Large Cognac brands are the reason why the region and its people survive economically, between direct and indirect jobs that the Cognac economy generates.
Without the main players, there would be none of the artisanal smaller guys. When one runs a large business, one's finance department's mission is to limit costs and maximize profits.
Cognac takes time. Time can't be hacked.
As large brands are buying their stock in bulk from hundreds of small makers, the variety is immense.
Some cognacs may have been distilled better than others. Some might have evolved quicker and more gracefully than others. Terroirs are different, some have been distilled at the estate in relatively small stills, others by professional distillers or coops in larger computer-run alembics.
To bring these various eaux-de-vies together under a brand/age designation that fits the company’s flavor profile, additives can't be done without.
Moreover, knowing that:
there is a legal minimum aging in barrels for VS/VSOP/XO of respectively 2/4/10yr,
different cognac from different provenances (the sub-appellations Grande Champagne, Petite Champagne, Borderies, Fins Bois, Bons Bois, Bois Ordinaires) are purchased at different prices (Grande Champagne being $$$ while the Bois are less so),
aging a spirit in a cellar immobilizes cash flow,
takes up space,
has a cost inherent to the work involved in caring for these barrels,
has possibly a cost linked to the capital borrowed to purchase those casks,
it appears logical that in a volume game, one will have incentives towards:
buying lower-priced distillates,
not aging cognac more than needed.
In that context, the great equalizers are caramel, sugar, and boisé.
Hence when used with the mindset of cutting corners, additives are likely not to improve the inherent qualities of a cognac much.
Granted, if the quality of the base spirits is poor, it might drink better after adulteration.
There is both an art and a science behind the use of additives.
Here again, two tiers:
Cognacs to which additives are added over a long period of time for better integration turn out better than those to which additives are dumped into 24hr before bottling. The better additives are married to the compounds that make up the cognac, the more pleasant the drinking experience might be. Take a small distiller who has a contract with larger brands or negotiant. The brand will tend to ask the distiller to keep her cognac in her cellar until the brand needs them, which is more cost-effective and easier in terms of storage for the brand. But the brand will want her supplier to store her cognac in specific barrels that the brand will send (and charge the distiller for) to the distiller’s cellar, assuring from the get-go a path towards consistency. Should one take the bond out of the empty new barrels and stick one’s finger through the bond, scratching the inside of the cask, one would often find one’s finger covered in a thin layer of boisé. Barrels have been filled with boisé then emptied, leaving a coat of the stuff that will give a color and tannin boost to the freshly distilled stuff. The use of additives over an extended period of time tends to lead to rather well-made cognacs.
Cognac to which additives are added last minute, close to the bottling date: like oil and vinegar, the different molecules are not binding, the nose is flat, the palate disjointed. These are the poorly made products that give the appellation a bad rap.
Now one should mention another layer of possible use of additives: those which have been aged, for years or decades.
It sounds counterintuitive, it is not.
For additives to marry well to the base of Cognac, they need to have had the time to bond with the molecules from that distillate.
For one who believes that additives might enhance the eau-de-vie (which is a thing) and would want to use additives to elevate their cognacs even more, they would take the opposite approach to using additives as shortcuts.
In some cellars, one can find the following:
sugar + water + cognac, the mixture at around 20% alc in a barrel,
boisé + water + cognac, the mixture at around 20% alc in a barrel,
water + cognac, the mixture at around 20% alc in a barrel.
Sugar/boisé/water cannot be aged for long periods of time without getting spoiled; this is why cognac is added so the proof of what's in the barrels is high enough to protect from bacteria.
This practice is rare as it's not improving the bottom line nor efficiency. To the contrary, it takes a LOT of cellar floor space and barrels to store lower proof 20% liquid than its 72% equivalent. It means cellars are filled with additives patiently aging in casks.
Some of these cognacs can be magnificent.
Inherently, additives are not bad. It is the intent with which they are used on most occasions that makes them so.
(*)Note that the argument for consistency is itself not as straightforward as it may seem. Some brands tend to look at markets/countries by flavor profiles, and it is possible that a VSOP might be darker or woodier or sweeter in one market versus another to fit with market research and what has been defined as the “palate” of a specific population.
*Boisé is akin to a wood-paste, a solution utilized in cognac production, derived from oak chips/fragments.
The primary purposes of boisé are:
Coloring: Young cognacs lack the deep amber hues characteristic of older spirits. Boisé provides an immediate color enhancement, making the spirit look older than it is.
Tannins: Boisé contributes tannins to the cognac, the wood compounds responsible for structure and mouthfeel.
Flavor: Boisé imparts a wide array of flavors that resemble those found in longer-aged spirits. These flavors can range from vanilla and caramel to toasted notes, depending on the preparation and aging of the boisé.
Rancio: Rancio is a highly-prized taste/smell that evolves in well-aged cognacs. It's often described as a rich, nutty, and sometimes mushroom or earthy flavor. Some boisé solutions are crafted to possess varying degrees of this rancio character, and producers can purchase boisé with a specified level of rancio.