I personally use them so I can context switch between projects. I can keep one group for project a, one for project b, one for project c, and so on while also keeping a group for day to day stuff, one for reading material, one for conference talks/background noise, etc.
Then I can just unload a given group when I don't need it without losing anything and I can bring it back up on that window (or a different window) later when I need it again.
https://addons.mozilla.org/en-US/firefox/addon/simple-tab-gr...