750 Words was backed by the idea that a writer had (the name escapes me, someone help me here) called Morning Pages. The idea was just to write straight, for either a set amount of time, for a set amount of pages, or a set number of words. 750 words is simply a decent number to hit, should you be writing a bit each day (though most writers will most likely hit more, should they be serious about it).
The general idea can be thought of as practice of a craft and technique, very much in the same way someone playing an instrument, say a piano, would practice scales. Yes, it's basic and habitual (which some writers will swear against
), but it can help writers loosen their creative muscles a bit, and help writers say creative and "fit", so to speak, with their technique. As an added bonus, usually writers can pull from whatever they're written in those 750 words, and use them elsewhere, should they see fit.
I think it's a great idea; practice is always a good idea. That, and it shuts up the editor in my head, so that's always a good thing.