What purpose do the tabs on roofing shingles serve?

I was recently asked this question by a friend & cannot find an answer on the web.

Public Comments

1. The tabs actually act as a guide to direct water off the roof. That is why you do not line them up on each row, but every other row... Plus it makes the roof look good....
2. Are you talking about a three tab shingle? If so it's just a look that people like, it gives it a tiled appearance. I always avoid using three tab shingles because they actually wear out quicker in the cut out for the tabs.
3. To channel water off the roof. Otherwise, it would drip and drain a lot longer.