Bookmarks I use for pages I want to store for a longer time. Tab groups I can use for pages I have open at the moment, e.g. because I'm working on X and Y, so I group the tabs based on X or Y. But I don't need to keep the tabs between sessions.

Scenario: working on two tickets and having some yt/spotify/whatsapp in background. I separate the tickets and tabs related to them in their own tab groups and the social media tabs in another group. I can easily navigate through them and have less mental overhead on where everything is, especially if I color code the groups.