Okay... You all are probably thinking "Duh, it's to shove your dick through whenever you need to take a piss." But how many of you actually do this? Reaching through that hole and fishing around for your schlong is not what you want to do whenever the tank is full, you just pull down and whip it out.
Now, let start off my theory of the "penis pocket" by asking a question:
How often are you just lounging around on your couch watching the game when you notice that your sack has decided to relax and your crotch starts to sweat just enough to stick to your legs due to your sweatpants?
And the solution? Grab a pinch of your sack and pull it through the hole. BOOM. Instant comfortable separation of sack and leg.
Thank you for listening.