More things that I don't get

Truck Balls. These aren't knew. I remember seeing them when I was much younger, but, in my innocence, I always thought they were some sort of counterweight balance for the truck hitch. It never occured to me that people would want their trucks to have artificial testicals.

Can someone explain this? The link says that they are:

a visual means of expressing the driver's masculinity

Huh? Can somebody explain that one? I'm really at a loss! Seems like a way of expressing the driver's juvenile sense of humor. You don't see women attaching artifical breasts to their cars, do you? Of course, some men might like that.