Riding is exercise and exercise requires rehydration. Even a half-hour spin can be too long without water. Those first few rides for someone not used to exercise can be strenuous and uncomfortable enough without having to go thirsty. Nearly the same can be said of gym shorts, or jogging pants. No, we don't wear bike shorts just to look funny.
Nothing can take the place of real bike shorts with the padded liner sewn into the seat and crotch.
Bike shorts are meant to be snug to prevent chafing and are worn by themselves with no underwear. Leave your BVDs or Victoria's Secrets in the drawer. Most look like regular casual shorts but still have that ever important built-in liner. Some even do double duty as water bladder carriers. One of the most essential items to carry.