Ever since the dialup era, when all we had was "open in new window", I would browse by reading down a page and opening interesting-looking links in new windows and letting them load while carrying on with the original page.

Tabs are a good LIFO stack of things which are currently "live", or at least not dealt with yet. It's easier to navigate than history or bookmarks. You could argue that at some point tabs should "spill" into bookmarks, but that requires making a decision.