Perhaps because you can’t view two tabs simultaneously?
Huh?
Usram has a good point (you can’t view two tabs simultaneously, but you can view two windows side-to-side), but why I’m looking for this kind of feature is because I usually put all currently open webpages related to a certain website (for example, the SDMB front page and several threads) in tabs in a single browser window. If I visit another site, I want it to be in another window. So let’s say, for example, that I follow a link to another site from the SDMB thread, and it opens in a new tab. Or, I use a tab that I don’t need anymore (say, a thread I’ve already read) to open a new site. I might want to transfer this tab to its own window, since it doesn’t have anything to do with the SDMB anymore. This is where this feature would come in handy.