The hook is used especially on snatches, where the grip is much more difficult. In the snatch, there is a tendency for the last and second to last fingers to bear a lot more weight than they do during the clean. The problem is that these fingers are weak.
I did high volume snatch pulls without a hook grip for several weeks and screwed up these fingers to the point where I couldn’t even snatch pull 60 kilos. They still aren’t completely tip-top. The hook grip reduces finger strain altogether and puts what strain there is on the strong fingers.
Oh, and BTW, when I hook grip I place my first three fingers on my thumb, and I always tape the thumb.