Once the keg has been gassed and the oxygen purged from the keg it can be stored out of the fridge. We do this ourselves as we often have to many kegs full of beer to fit in our fridges.
However the beer will age much faster if it is kept in the fridge and I will explain why.
Beer that is consumed early is often described as green. What you actually taste is the live yeast in the beer. As yeast is a living organism it needs three things to survive, Warmth, Oxygen and Food. The beer has a ready supply of food; we call it our beer, so that leaves the oxygen and warmth to remove. We have removed the oxygen when we pumped CO2 into the keg and purged the beer so all we have to do now is remove the warmth by putting it in the fridge. By removing both the oxygen and warmth from the beer the yeast dies faster and the beer ages faster!