It's actually because I'm so terrible at remembering certain types of things that I keep so many tabs. They are essentially an accumulation of "what I need to work on" for the next few days or weeks, along with the supporting research work I've done to accomplish those things so far. By grouping them in separate windows per task I'm basically keeping all of that context. I could also do the same with bookmark sets, but I've found when I do that I tend to forget about the task itself entirely.

There are probably more efficient workflows, but this one works fairly well for me.