You mean in GSM? Firstly, you must understand that just because GSM allows you to choose those modes, it doesn't mean that whatever software you boot afterwards will display stuff on the screen nicely.
The only sure way to have proper and full HD widescreen support is to code the software to support those video modes.
As for why 720P and 1080P are included... I'm honestly not too sure myself since GSM cannot increase the resolution of the frame buffer (You'll really need magic to do that).
It was discussed before, and the most feasible progressive-scanning modes would be 480P and 576P (For NTSC and PAL games appropriately). I think that people have claimed that using a higher resolution (720P to 1080I) eliminates the problems that they had with upscaling done by their TV sets.
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DESR-5100 S. EMCS