You give them to the kirbir fund! lol just kidding, you can either buy sigs/tutorials etc, or give them away if you want, entily up to you
Me personally, I don't reallly need or yuse my creds. If I see a tut or sig I like I just buy it. Otherthan that its nuthing.
i dont buy anything ever... if i REALLY want something, i can get it or something better from someone else for free, so i have no need. I usually save them up till i get about 10-15K (between 7-14 days when I have a new tutorial out or when I get off my butt and start making mass signatures... about 5/day for about 375-550 a piece) and sell them for money, that and once in a while I will donate them to some random people who really want a sig or something in the auctions forum but dont have enough money for them.
Well you want to get more so you can say "Ha, I have more credits than you", but say it to someone that's not very bright so they feel bad even though credits dont really do anything. ( mine do tricks!)