Why is the scrotum wrinkly and hairy?
It has to expand and contract to keep the testis at an optimum temperature to produce sperm.
… and it’s hairy because it has hair on it.
Asking this question must have taken a lot of…
…no, I can’t do it, too easy. 
Because if it was smooth and white it would be an aspirin.
They are?
Its because it doesn’t get out more.
Cecil has written about the hairy part, here.
If girls fondle them long enough they get smooth and silky.
Thats the story I told my beloved 30 years ago and I’m sticking to it. 
Thermal management. As sitchensis suggests, there is an optimal temperature for sperm production. Scrotal skin tenses and relaxes in accordance with temperature. You can see this happening in certain situations:
-sit or lie as still as possible and just watch your scrotum. You’ll see areas of skin slowly contracting or relaxing, responding to minor temperature variations.
-Spend ten minutes in a hot bath. Get out, and observe that your scrotum is completely relaxed, hanging as low as possible. It’s too hot, and your scrotum is trying to maximize surface area in order to dump heat. Obviously this isn’t helpful while submerged in hot water, but your scrotum doesn’t realize this (and it works pretty well once you get out).
-Spend ten minutes in cold water, or go for a long walk on a very cold winter day. Or if you’re bold/impatient, apply an ice pack directly to your scrotum. Observe that your scrotum contracts tightly up againts your body. It’s too cold, and it’s trying to minimize surface area in order to restrict heat loss.
You’re saying, if your hands are cold, you should cover your balls?
It is not only surface area, but also distance from the trunk where body heat is generated.
OK, I believe you will enjoy this:
You mean the rugae of the dartos?
Cf. Where does your lap go when you stand up?
ETA: knowing “philtrum”'s got nothing on rugae, I think.
Okay, why does it have hair on it? If it needs to be cold for maximum sperm production, and hair retains heat, why is it hairy? Seems counter-productive.
If thy wrinkly, hairy scrotum offend thee, pluck it out, and cast it from thee: for it is profitable for thee that one of thy members should perish, and not that thy whole body should be cast into hell.
My guess would be ancient genes. Back in the day, we all went around naked, and mostly covered in hair. On both sexes the genital area was important to the continuation of the species and developed extra protection. I think that the male organ was generally much smaller then than now and the hair probably more luxurious.
Well, I’ve told my beloved of 30 years that it’s the best hand-warmer you can find. She seems to agree. 
Ah, the “shrinkage”.
(I’d post the applicable Youtube link but I can’t Youtube here. Google “Seinfeld shrinkage” if you need to. It’s the first two returns at the moment.)
Hair doesn’t grow out of my scrotum, it sits on top like a bad toupee.
I tried once to wax my scrotum, and it was the most painful thing I ever did. Is there any other way to trim our bush without excruciating pain or having a blade rubbing against our balls?