Presumably it's useful for web developers on Windows 7 or above, but who need to test on IE6/7.
As to why they'd use their own VM in preference to VirtualBox I don't know. But to be fair, Microsoft isn't the only company that neglects developers who aren't on their platform; "not invented here" syndrome seems to impact all software companies eventually!
As to why they'd use their own VM in preference to VirtualBox I don't know. But to be fair, Microsoft isn't the only company that neglects developers who aren't on their platform; "not invented here" syndrome seems to impact all software companies eventually!