If I had to guess, it would probably be to increase your hand's friction. That being said, you need to remember that not everything in evolution necessarily has an advantage. Some things happen as a side effect of other changes.
The most classic example is dogs. They evolved from wolves and changed their appearance almost completely in a very short period of time. That's because the genes that made them less scared of humans also happened to effect the color of their fur, the shape of their heads, their sizes, etc. By mere coincidence their appearance was changed almost entirely.
(Of course, these days many dogs are bred to look certain ways. We're talking about stuff that happened pre-selective breeding.)