They are really just for looks as they have holes in them to let the corks breath. They do however seem to usually hold a cork in if refermentation occurs. Dont ask how I know!
I also try to match capsules to labels. When I go to my supply house I usually try to buy an assortment of 100 each so I have them. I pay around .05 each for them at that quanity.
I believe that I read somewhere that the caps on wine were originally made of tin or lead. They were placed on the bottles to prevent rats or mice from chewing at the corks. - Yet another useless bit of knowledge -
Dude you really think I haven't checked it out already. I can even go with tin. Problem is I don't want 5,000-10,000 of one color. They are really cool though as you can use up to 7 colors in the capsules and have the top embossed as well. http://www.lafitte-usa.com/capsules/