As a man with good personal hygiene who also doesn't play with his balls, I can offer only an academic opinion. Which is: testicles are vulnerable. When as a child I lived briefly in Barcelona it was common to see men strolling the Ramblas playing "pocket snooker" and spitting garlic onto the hot pavement. Men seem to have a visceral need to reassure themselves that their balls are still intact and safe from harm. Indeed, this procedure of ball-cupping is common across every known culture, and often associated with superstitious/religious rituals that supposedly ward off evil. The English language carries the fossil of such beliefs, as the word "testify" means "to tell the truth while holding one's testes as a sign of veracity."