I can tell you exactly what made me use tabs instead of bookmaking sites: opening a bookmark means making a new connection, new round-trip, loading, parsing and rendering a full page. Having a page open in a tab means it's already there, in exactly the state you left it in. It will still be there if the Internet goes down. It will still be there after it comes back up. It will be there tomorrow, it will be there next week, after hibernating and waking up your computer a dozen times in between. It will be there even if the page is already removed on the server.
The difference between a tab and a bookmark is like between a book on your desk vs. in a public library. In the latter case, when you need it, you have to go there and fetch it - not only taking time, but also possibly failing, as it might not be there anymore.
Is it a perfect interface? Not really. What I'd really like would be this, + in addition a bookmark feature that actually freezes and snapshots the page I'm on - to get the same behaviour as tabs, only persistent. It shouldn't be hard for browsers to do, given that IME at least some browsers displayed this behaviour at some point when restoring pages after browser crash.