You answered your own question, seeing as you found that it's to do with video memory mapping, that's what it's needed for .. remapping the memory area for video memory.
.. and you didn't look any further into the function of it, just finished at the first line of it's explanation ! There is more to it, but it's slightly complicated, so I'd suggest reading more about it ( in your own language if possible, I'm sure someone has translated it into nearly every language on the planet ).
For starters, you can read about it here :
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