There's a button on the top-left which collects the current tabs in the window and puts them aside. They can be restored later.
Personally, I've only used this by accident. But I could see it being useful for "I know I want to come back to these tabs later (e.g. tomorrow), but I want to close the browser / do other things now".
That sounds like a way to save a browser session that comprises of tabs. Perhaps a better UI. Many browsers have had the ability to group/tabs/bookmarks. But it's usually quite clunky.