Smooth animations and performance. Draw a big image with Win32 BitBlt it's painful slow, for example. Imagine that you are zooming an image in Photoshop and it is laggy, the user experience would be horrible.
Also, the lag is an important issue in the user interface, even something so small like 100ms would be bad.
Why not just use DirectX/*GL for those regions that need it and stick with platform UI for the rest? Blitting API still works just fine if you're drawing combo boxes, no?