This article is from the Real Ale FAQ, by with numerous contributions by Brett Laniosh others.
A: When a cask is being emptied over a few dozen hours Carbon Dioxide
(CO2) is released from solution in the beer into the space above the beer.
However this is a gradual process as during opening hours the spile hole
will be open to atmosphere and there is plenty of opportunity for the
ordinary atmosphere from the cellar to enter the cask as the beer is
tapped faster than CO2 comes out of solution. If, instead of letting the
cellar atmosphere and whatever pollutants, organisms and oxygen in, why
not help the natural process of CO2 release along by piping CO2 to the
spile hole? In the more brutal system this involves CO2 at more than
atmospheric pressure. A correctly set up cask breather system maintains
the pressure in the head space at atmospheric pressure. This sounds ideal,
but there are arguments for against and alternatives. The argument for is
basically that the beer keeps longer and the end of the cask has a better
head. The arguments against are basically: Even at 1 atmosphere the beer
takes up much more CO2 and the sterile atmosphere prevents ageing. The
alternatives are basically: If there is no problem then do nothing. Plan
to use casks in about three days. If demand isn't present then don't stock
it or use a smaller cask size. Use nitrogen.