Two things should be mentioned first:
- Butt hair is not common in everyone, but it runs in some lineages.
- The human body was never wired for convenience. It might be some odd evolutionary addition that was never really removed, something that was once of benefit, but as we evolved, it became quite useless just like the appendix.
The original function of butt hair is protection. You must never forget that every hole in your body-including your butthole are quite sensitive. So like every other sensitive hole in your body, there is a need to protect your butthole. At our current evolutionary position, we stand upward in a straight posture. In this straight posture, the muscles for sitting convenience completely fold behind it, giving it all the protection it requires as long as we ensure it is clean. But before we acquired this posture, we would have ended up with a posture like that of the gorilla. Try to bend over when your pants are off your ass. You will discover that the both sides of gluteous maximus are no longer completely sealed. So imagine you are in that position at all times, everyday.
If this were to be the case, you would have had your butthole exposed to open air, sun, and even parasites. Having hairs in your asshole in such circumstances would make the hair a kind of insulator and sunblock and prevent the small critters you hate having around that little hole from making it their permanent residence.