When dispensing keg draft beer, the goal is to keep the CO2 level prescribed by the brewer. Any change in the CO2 level will alter the taste, pouring characteristics and appearance of the beer. Most breweries in the U.S. recommend a CO2 pressure between 12-14 lb.
Too Low Pressure
If the draft beer is dispensed with too low of a pressure, the CO2 that is dissolved in the beer will "break out" of the beer. Initially this will cause the small bubbles of broken out CO2 to float up the beer hose which will result in foamy beer, and over time the low pressure will result in flat beer.
Too High Pressure
If the draft beer is dispensed with too high of a pressure, over time more CO2 will be absorbed into the beer causing foamy beer.