1) The cooler it is, the more buffer/time you have to deal with a temperature issue 2) There is a thermal gradient between the hottest parts of the system and the cool ambient temperature. The cooler it is, the quicker that heat is removed. For every degree in difference, the cooling effect is increased/decreased.
That said, I wonder how many people actually know what the optimal should be and how many people just think that, hey, 15 degrees C should be OK!