What Pyscho said.
The main benefits of Pinpics are three-fold, really.
1.) It's the biggest pin database available to the public. It lets you see retired pins, as well as new releases. It's our encyclopedia.
2.) It allows you to create accounts, and divide your pins into three sections- your collection, your Wants, and your Trades. It's a great organizational tool for keeping track of your pins.
3.) It's an online trading system. Once you set up an account and add pins to your Trades and Wants, Pinpics can match up potential trading partners. Mass e-mails get sent out when you kick off a trade request (they usually go out in bulk around 6 AM EST). If the person is interested, they'll reply to your e-mail and work out details with you.
It has other nifty functions, too, but those are the main reasons we use it and refer to it so often. I can't imagine pin trading without some sort of database...I wouldn't know about half the pins I do.