three purposes: 1. It makes it possible to run the game in "pseudo-fullscreen" mode on one display and use other programs in another. No need to see that window border all the time. 2. It makes the game handle popup windows gracefully - instead of switching resolution and minimizing the game the new window will simply show on top of the game window. 3. It makes it possible to run the game in high resolutions, even when the game itself fails to detect them.
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