Most of these come from sites like this one. I will go down the home page and open every article I want to read as well as its comments page (middle clicking). Thats easily 20ish tabs. These tabs will then stay open most of the day as I slowly read them.
Then I have some that are used sort of as a todo list. These can be a page where I need to fill out a form, read an article, etc.
I also have a bunch of tabs based on a group. For instance since I do a lot of web programming I may be working on 1 site and have 1-8 tabs associated with it, but then I need to switch to another site and open another set for it, but I don't close the previous until I am done with the site.
Then I have the normal "apps", pinned tabs or whatever they are called. Usually just 2 though, gmail and grooveshark. However sometimes it grows a bit more to add calendar, docs and whatever is is needed.
Add it all up and its quite a lot of tabs, and they just add up quickly. But with the ability to place tabs on the left of the screen and group them its easily manageable. I couldn't work with this many tabs without those 2 features.