Gum tissue, like skin, protects the tooth and bone from injury and bacteria. A minimum amount of attached gum is necessary to protect and maintain the health of the tooth.
Most gum recession is caused by tooth position relative to the jaw and not by over brushing. Take a look at the teeth in your own mouth. Those that are receding are most likely to be those that have been pushed closer to the lip or cheek (either grew that way or had to be placed there during orthodontic tooth movement).
All gum recession in the adult is also a loss of bone. Therefore, recession is much more than a cosmetic problem!