Overheated
As I sit naked in front of this computer my balls rest on the inside of my thighs. Then, as I gat up, I notice how low they hang. They must be trying to get away from the heat of my body. We all know how balls like to be a few degrees cooler than body heat and dangle accordingly. BUT what if they were hemmed in by underwear. There would be no escape. They would get overheated.
I think there's been quite a bit of research into that area relating to sperm production. It may be important when it comes to making babies, less sure for other activities. A cock ring creates the opposite problem by holding everything out when cooler temperatures may mean the body wants to retract! The forces seem to be gentle, so there's no discomfort from that aspect. Skin irritation is the greatest risk for me, it seems.