That is a fantastic idea. In my case, my tabs are split between tabs necessary for attacking a freeCodeCamp challenge (maybe 2–8 tabs including the page with my solution running) and maybe 3–5 Medium or other articles on coding I want to read, but are heavy enough it requires serious thought like on Promises/A+ or functional programming or whatever.
If I set aside the second group of tabs, I rarely get back to them. Same if I bookmark them. Ugh. (And I gave up on bookmarks decades ago mostly.) Stuff that goes into Pocket disappears, too.
I wish Opera let me stack tabs so I could save an entire group at once.
I also may have 6–12 tabs for some other outstanding TODO or research project, say a new kitchen appliance or whatever.
So my tabs split between active tasks and nice-to-read articles. And running separate browser windows for this is often awkward too.