The most typical case is that I am looking at a list of things, and I want to click on a result but if I hit back, I don't want to wait an eternity for the page to reload -- if it even does; more often than not something breaks in the back navigation and the results I was looking at are not longer available or the context has switched.
Basically I want cost-free backwards and forwards navigation, and tabs are how you do that. But essentially I want that hidden from me. I would much prefer if under the covers hitting back didn't navigate you away from a page, it just keeps the page you are on in a virtual tab in case you want to go back to it. Similarly, clicking a link shouldn't navigate away from the page you're on, but just put it in a virtual tab.