Balls. When one thinks of penis, one thinks of a stick AND a pair of balls below it. A penis without balls is just a meat stick, a bad excuse for a limb. However, a ballsack serves a greater purpose and is instantly recognizable. People would much rather suck a ballsack than suck a cock, I can guarantee that much.

Besides, a cock can be easily replaceable. Ever heard of a strap on? Balls are so unique they cannot be forged by the mere hands of man. Everything in existence pales in comparison to a perfectly crafted ballsack.

I rest my case.