A lot depends on your physiology and the temperature of the room. I tend to run "hot", so I don't need legwarmers as much as others. But our studio can also be pretty cold sometimes. In that case, I have a pair of legwarmers to keep my legs warm.
Sometimes dancers will wear legwarmers over an injured body part to try to help it heal and prevent further re-injury.
Maybe you could borrow a pair from a friend one day and try them out. If you like them, then get your own; they can be pretty cheap mail-order. If you see no purpose after trying them, then at least you've answered your question.
Hope this helps.[/i]