Originally Posted by Towlie

The cycling shorts hold your boys up and out of harm's way (as well as possible). Some also claim they hold your muscles in position. The padding is important for relieving discomfort, although with some of the most comfy saddles it would be theoretically possible to go without. They are also more aero.

I've backed off from jerseys recently. They also make you aero, allegedly help whisk away sweat, and prevent flapping in the breeze. But lately I've opted for a t-shirt cause they're more convenient, more abundant in my dresser drawer, and less geeky looking.