Think about Cosmetics as skins for armor parts or weapons that you already have.
Keep in mind that you might receive a Cosmetic even though you do not have the armor or weapon that it is going to be applied on. To apply the Cosmetic, you first have to receive the item.
The Cosmetic will only improve the looks of your armor or weapon, there is no stat improvement.
Also, applying the cosmetic skin over the item does not change the appearance permanently, if you liked the old look better, you can deactivate the cosmetic any time by going into the inventory, clicking on the item and then clicking on the old appearance. This action does not delete the cosmetic and you can reactivate the cosmetic at any time you want by following the same steps.