The purpose of this is to annoy the victim while providing amusement for friends or bystanders within the confined area. If more than one person joins in on the proceedings, it becomes a crotch zombie swarm.
The purpose of covering the actual crotch with one's hands is to provide a physical bulwark of heterosexuality, or no homo.
"Yes, son. I know it's sad, but that's just how life is."
"What's that thing sticking out of it?"
"That's a crotch zombie. Look away, son. Look away."