The documentation states it's a blocking call. There are reports of it being used for WaitForVBlank-like purposes [1]. Without blocking Softube VST plugin logic breaks and they never update their UIs.
[1] https://news.ycombinator.com/item?id=34501612
Wine-Bug: https://bugs.winehq.org/show_bug.cgi?id=56935
-- v2: dwmapi: Simulate DwmFlush blocking behavior.