Just to give an example how my tabs accumulate:
I regularly read gmail, and I like having iGoogle open to check on my feeds. I have the main Steam Gifts page and the forum page open, as well as the GOG forum, the Double Fine Adventure forum and the Wasteland 2 forum. I have the Kickstarter pages for Spate and Jane Jensen open because they're the active ones I'm interested in. Trent Oster's twitter might have something interesting about the Baldur's Gate remake, so what not keep it open? Actually it's kind of boring, so I might close it one day. I keep Stop Forwarding That Crap To Me on YouTube open because I enjoy listening to it occasionally. I have Cucumber Quest open because the Kickstarter introduced me to that and it's fun, and I should continue reading it sometime. I have the Lineage II site open because there's a problem there preventing me from getting the anniversary gift and I need to follow up on that before it expires.
And so on. A lot of times tabs get left behind which aren't relevant, so I'll delete them if I come across them (I deleted a group of 14 tabs while writing this), but basically anything I want to check on later on I keep as a tab.