Originally Posted by LordOpie
your testicles can be damaged from the cold and are designed to move in and out of the body as needed to regulate their temperature.
It's actually less that they'll get damaged and more that sperm production peaks at a slightly lower temperature than the body's core 98.6 degrees (37 degrees to you wacky metric users). Therefore the scrotum is designed to drop the testicles away from the body on hot days so they don't get too hot, and pull em back in on cold days so your nice warm body can keep them at optimal sperm production temperatures.
It's all about procreation and passing on your DNA. Everything else is just noise.