Brushing in the evening is extremely important in order to remove all acidic compounds that are the main force behind tooth decay. Such compounds result from foods and beverages consumed during the day, but to a lesser extent they are produced even by air oxidizing you saliva. Brushing after each meal is recommended, again to remove the acidic remnants and any food debris that may be left to rot. Brushing in the morning helps to remove acidic compounds and bacteria commonly resulting from the stale environment generated throughout the period of inactivity.
While it may seem that the recommendation reads as "more is better" please also keep in mind that while brushing you are also damaging the enamel that is meant to protect your teeth from all of the acid, bacteria and resulting decay. The degree of damage depends on your toothpaste and the mechanical force you exert while brushing (so many gentle brushes is always better than fewer, harsher ones). If at any point you start feeling sensitivity in you teeth you are probably overdoing it.