Originally Posted by Lars Honeytoast

The way most saddles are curved, they are meant to be installed with the nose slightly tilted upwards. So you butt gets seated in the middle of the curve and you are not fighting against your own weight on the handle bars, and to put an even amount of pressure down on the center. Enough people come into the shop where I work complaining of comfort issues and one of the first things we do is check if the saddle is flat.

Saddles that should be adjust to a flat level are usually for sportier bike and the such