Biologically, the testicles produce semen and some other hormones for the male gender. To human males, the testicles serve as mobile playthings.
The testicles like to be warm. That's why on cold days or in cold water, they shrink up to be closer to the body. This temperature regulation is to keep the testicles at the optimum temperature to produce sperm. No one thinks about that though that's why we all put several socks in our Speedos at the beach.
The testicles are where sperm is produced. Assuming that anyone reading this is able to talk a woman into bed and missed the day of sex ed when they preached the use of condoms to prevent babies, during ejaculation, a man will release millions of little sperm into the woman. The sole purpose of the sperm is to fertilize an egg and nine months later, a baby is born.
Hopefully this one doesn't make it.
For most readers of Cracked, 99.9% of your sperm is released into tissues where they will swim around until they suffocate looking for that fucking egg.
Stay with me Joey, the egg is just a little further.
One final thought, according to scientist that study this sort of thing, the penis is shaped to displace other mens' semen. Think about this the next time you're the caboose in a train.
The testicles are the source of male fertility and fertility equals power. That's why when someone is particularly brave, it is said that "he has big balls" or "that was a very ballsey move." Conversely, when someone is a coward, it is said that "he has no balls."