People can use a ton of tabs for a ton of reasons. I used to do the same thing at times, when I used Treestyle tab in Firefox. But it's an old type of extension soon to be deprecated, and was giving me bugs on browser updates (maybe due to it being a XUL extension).
The add-on put tabs on the side of the screen, and grouped them up into trees, which you could also collapse. So if you were to browse Wikipedia and click on all sorts of links, you'd have a whole tree with different branches. Then when you take a break, you just collapse the tree and start a new tree. Want to reference something from the Wikipedia tree? Just expand it again!
It worked really well with the session restore functionality of Firefox, which doesn't reload all tabs again when the browser launches. So you had very little performance impact for what felt like the perfect bookmark and tab integration.
So in order to reach 100+ tabs, it's just one hour of casual browsing per day and within a week you'll have 100 tabs open of Wikipedia trees, various clothing stores, a code problem tree or two, and a HN tree with all the little interesting links.
Then you may wonder why you wouldn't just bookmark pages and close tabs? Personally I open bookmarks a few times a year, but this Treestyle extension made me click around so many times per day.
(By the way, if anyone has found a similar add-on please let me know)