When a brand puts a cosmetic product on the market, they need to declare the main ingredients of the product and certain allergens when they exceed a certain amount in the product. The current list of allergens contains both natural and synthetic materials. The natural ones can enter a product as part of an essential oil or other plant extract, or they can be direct additives to the formula. Essential oils are mixtures of chemicals and some of them are not nice for the skin, so they are regulated* The brands need to declare them because some people are extra sensitive (aka allergic) to those materials and so they might experience irritation, redness, or rash regardless of whether the material is within limits. In this case the user of a product can go to a medical doctor who can find out which material is responsible for the symptoms. The user then can avoid using product containing that specific material when they see it mentioned on a label.

*In some cases, natural extracts contain materials that are so harmful that they need to be removed from the essential oil so as to make it safe to use in a cosmetic product. For instance, citrus oils contain so called furocoumarines that turn into peroxides when exposed to sun light (called phototoxicity). If those materials where not removed prior to adding them to a product, then using a cologne in the sun would cause serious skin burns for instance (this is the reason why you can bleach your hair by applying lemon juice on it and go sit in the sun for a while). Another example is the atranol found in oakmoss absolute that will give you rashes and severe skin irritation, which why now all oakmoss is low atranol).