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think that the thing about crawling is that they get spacial relations from crawling. They learn the very basics from my face is this close to my hands, it takes this long to get to here, etc. And the distance from hand to face while crawling correlates to the distance between where they have their head when writing later on (or something, I can't remember if its writing or something else LOL).
I'm not sure if the reason is that stated above, but its something similar. My friends is a primary school teacher, and she "teaches" her grade 2 class to crawl every year, as she found that some kids have never done it as babies.