Why do i need to CreateCompatibleDC while drawing dwm extended frame?

[EDN Admin]

While following the tutorial http://msdn.microsoft.com/en-us/library/windows/desktop/bb688195(v=vs.85).aspx
Custom Window Frame Using DWM (Windows) , the code shows that text need to first draw on to a memory compatible dc then BitBlt back to the screen dc.

But Why?
Thanks.

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