For example, say I'm booking a trip. I always open a bunch of sites (Kayak, Booking.com, lots of hotels, Google Maps, places to visit, etc.) in a single window. In pre-computer times it would be like covering a desk with a ton of papers, books and notes. Gradually I will figure out stuff, book the trip, etc. but the tabs can stay for quite a while.
I feel like many "dozens of tabs" windows are little projects like this. For example, doing development I typically have a bunch of documentation tabs open. We keep these windows open because there's no way to stash them into a drawer while they're not actively being worked on.
What browsers lack is a good way to treat these tabs as "persistent workspaces". I'd like to be able to close a window and be able to return to it later. Rather like an IDE which remembers your open files. So I wish I could "save" a window (as a "workspace") under a name, after which every action would automatically update the saved workspace. Close the window, workspace stays saved. Open the workspace, everything is restored.
There are some browser extensions that allow saving groups of tabs, but there aren't any that behave like I described above.