When DevTools are launched docked to a side of browser, that button near closing cross would minimize the DevTools panel, just so the tools tabs are visible. At least when it’s docked to bottom, but side dock could also minimize to a {90deg rotated / icon-only} tab strip. Clicking on any of them would make the panel the same size as it was before minimizing and with that tab opened.
That would be easier than resizing it manually, since resizing activation area is only like 4px big, and there are many cases when you need to see what’s behind dev tools better without closing it (whether it’s waiting for a breakpoint in Debugger, or you need to keep Console context, Network tab rows, etc)