Whatever you call them....man parts, berries, family jewels, nuts (giggle), they're the sperm production factory! They hang like they do for practical reasons - to keep sperm at the optimal temperature for healthy development - generally 1-2 degrees cooler than the rest of the body.
You've probably heard from someone that if you're trying to make a baby, that boxers are best.
Rest easy men, your tighty whities don't raise the temperature of your sperm factory. There have been many studies looking the differences in potency and counts between boxer and brief wearers, and there is no difference; wear what you want.
However, note that heat does make a difference, so limit your exposure to jacuzzi's, steam rooms and saunas. If you sit the majority of your day, try to get up and walk around every two hours for 10-15 minutes. If you drive, walk around every two hours as well and keep the vehicle at a comfortable temperature (and refrain from those heated seats!). If you need a protective cup for sport, limit its wear to less than 6 hours a day, remove it immediately after you complete your exercise, and make sure to keep well hydrated! Also, if you have excess weight in the groin area, your body will be warmer too due to the extra insulation - if you're ready to make some babies, it's a good time to loose some weight - just don't go overboard because more than a 2lb per week weight reduction can shift your body into an internal energy crisis and resources go to immediate survival versus reproduction!
Doing prep work to have a baby isn't just the role of the woman. It's a team sport. Super Sperm make Super Babies.