meh, I've heard some people say it helps organize stuff when you wanna keep a lot of tabs for something like projects or whatnot but I don't really use them in Vivaldi either
For a limited set of desktop circumstances (the music tournament, for instance) where I need a couple windows to act as a block, it can be useful. But it also tends to try and group things when I don't want it to, so it can also be a nuisance.
I've never even opened more than 20 tabs in one window (that I use/need for more than 1 hour),
and only a maximum of 4 browser windows.
So that feature is definitely not for me. But I'm happy if the feature is useful to you.
Personally, I often use the mobile tab group feature on chrome mobile when i have a lot of chapter to catch up. I can't use Mihon and stuff because my phone is out of storage.
On desktop however, i don't use it that much... (only for developpement maybe).