They are mostly the same as keeping them in separate browser windows, but with the advantage of being in one browser window. They also have the advantage of being label-able.

I don't keep tabs open forever, but back when I used Chrome I regularly used tab groups when I was working on multiple projects simultaneously; the Jira ticket, the PRD, the API documentation, the necessary AWS consoles, and the GitHub PR for one project go in a tab group. Name that group and collapse it, and now you can easily reopen it again when you're ready to switch contexts.

"Why not just put them in a separate window?" Sometimes that's preferable. Sometimes both solutions together are better. On a single monitor, having everything in one window is usually preferable for my workflow. In any case, you can't name a separate window. And if you use a sidebar extension, they aren't persistent across multiple windows.

"Why not just use bookmarks?" Bookmarks are a long term solution. Tab groups solve the short term problem. They're ephemeral.

"Why not just close the tabs until you need them again?" I do that as much as I can. But it's not practical in all cases. One project is in active development, one is in PR Review, one is in QA, and I have a support escalation I need to work on in the meantime. Each of those tabs might be needed at any time during the week.