I'll start by saying that I'm a web usability expert of 10 years.

From a usability point of view, the answer is simply that some people prefer navigating differently, and so the software has catered to the preferences of some users. It's not that all people prefer tabbed browsing, but that it's another option which allows a particular mode of navigation by certain users.

My guess is that firefox (who pioneered this) identified the fact that many users will navigate in a series of stages, being

1 - SEARCH - search for the subject in google (or whatever)
2 - COLLECT PROSPECTS - open each of the links to the pages they are interested in
3 - FILTER TO SHORTLIST - review each of the pages to see which are useful
4 - READ - once filtered, read these pages in more detail.

This type of structured search process is more common amongst very experienced users such as professional researchers, as it allows you to 'switch' from one mode to another, applying different levels of editorial filtering and concentration at each stage.

I cannot provide any references for this, it's simply original thinking based on my experience in the field.

If you were to observe users performing this process without tabbed browsing, you would notice that it is very difficult for them to break off the process at any point, as they have to save each favourite seperately and group them manually. With tabbed browsing, the user is able to save all the current pages at any point, and so they are freed to break off the process and return to it later.

Tabbed browsing also works well for user bahaviours relating to the use of 'live marks' - RSS feeds which allows a group of pages to be opened simultaneously in tabs.

Finally, there is also a technological reason. Now computers are much fmore powerful, users are able to open more and more web pages at the same time, and so better management features for handling large numbers of open windows are increasingly in demand.