Cuz I use them as a way to keep tabs (heh) on different projects I’m involved in. Tree tabs are much faster for me to organize into folders compared to bookmarks since they’re already part of my flow of using tabs in the first place :)
That being said, I end up using them more as a way to search through pages I had opened before, using the URL bar. Browser history is a little more finicky to search in that regard
As for how many I can close, I tend to close tabs once I’m done with something in a project (though some tabs I keep around if i find them to be useful beyond that specific project). I also have a bunch of tabs open for music and videos that I want to share with my friends when they get time which could be closed once I share them