I think a lot of pet supplies still fall under the 'buyer beware' category, unfortunately.
But supplements can be good for some cats, in some situations.
My oldest cat gets an enzyme supplement called Prozyme to help with her digestion, and I used to give my RB cat Cosequin for her arthritis. There are also fatty acid supplements for skin troubles, and vitamin supplements for pregnant and nursing cats, and for pets recovering from stress, etc. It's always just better to check with a vet & make sure it's OK before giving anything out of the ordinary.