Because there's a lot of lame coders out there who think they're just king s**t because they can make a webpage in Dreamweaver and they can see it in IE. Some never test it with other browsers, or either aren't good enough to make it compatible with others, or just plain lazy.

Other times, IE is just required, such as in Windows Update (if you don't know how to save and execute locally).

Bottom line -- this will enable more people to ditch IE completely (aside from its inclusion with all Windows builds) and run thier browser instead.