Using a hook grip serves a few purposes:
1) Prevents muscling up the weight by making your grip on the bar better without involving your biceps and forearms as much.
2) For some, makes it easier to flip their elbows under the bar because it becomes easier to rotate the bar/arms forward.
3) Cuts out having to do extra grip work when the weight gets heavy.
I used a hook grip for a while until I got the hang out of getting under the bar. After that, I switched to a regular overhand grip for much the same reason that I like to jump up to the chin-up bar with weights on my belt...