Evolution can be pretty stupid sometimes. It works pretty much by brute force, sometimes literally, so it ends up taking strange routes. Remember, evolution is not guided, not stupid or smart, just a natural process.

Humans make the mistake of assuming that we are the pinnacle of evolution, and therefore every trait we possess must be of benefit for some reason. In fact, we are not the pinnacle of evolution, and we still possess many traits that make little sense from a "survival of the fittest" standpoint.