On Chromium, when you're in a tab group you get a narrow toolbar on the bottom to quickly swap between tabs or even close tabs. When looking something up, you can pick 2-3 pages in the results to open up in the group and flip between them quickly to compare and build consensus. Without that on Firefox, switching between tabs takes 3 taps and it's really annoying.

In addition, tab groups end up filling the same function as separate browser windows. They server as a way to scroll through tab topics in a very condensed manner, since I can have one tab group for Skyrim mods, another for gardening, one for 3D printing, etc. So then you scroll through topics instead of having to scroll past 20 tabs on one topic, 12 tabs on another, etc.

What I personally don't get is why Mozilla insisted on implemented them on desktop instead. Tab groups are just redundant in that application. Maybe someone else finds a use for them (and that's good), but it'd be better for them to fill out a missing niche first before adding more.

Also, I've noticed some people recommend Firefox's tab collections. Those are not the same thing. They're just glorified bookmarks.