Not the person you're replying to, but may as well answer since I also use tree-style tabs similarly.

> - Why keep a site open when you are done with it?

Why assume you're done with it? I have a number of tabs just for Grafana dashboards I'm monitoring. Re-finding the particular configuration from the home page every time would be annoying, bookmarks would require reloading the page (and thus re-running the slow queries that happen on load instead of on data update), etc.

> - Too many tabs and it gets difficult and irritating to find and open the site you want.

That's what tree-style tabs solves. Tabs are a hierarchy, just like bookmarks or folders normally are. It's much easier to navigate.

> - Have never needed to keep many, many sites open in the first place.

I've got email, calendar, a bunch of Grafana tabs, plus a few datasheets, a schematic, Jira, Confluence, and Github.

And that's just for work, for news I tend to open articles in tabs with HN comments in child tabs and then go through & read them in sequence, but I keep work & non-work in different windows.