When dispensing draft beer, the goal is to keep the CO2 level prescribed by the brewer. Any change in the CO2 level can alter the taste characteristics and appearance of the product.
The recommended CO2 pressure, for most domestic draft beers, is between 7-10 Psi . This pressure will maintain the level of carbonation that the breweries specify.
If the beers is dispensed with too low of a pressure, over time the CO2 that is dissolved in the beer will break out. This will result in flat beer. If the beer is dispensed with to high of a pressure, over time more CO2 will be absorbed into the beer and the result will be off-taste and foamy beer.
Stout beers such as Guinness, require a mixed gas and a higher dispense pressure for proper dispensing.