simple physics, the cooler it is then the less wear and tear. drives last longer if they are consistently kept at a low average temperature. there are cases of hard drive failure as a direct result of overheating. it's like having a heatsink on your processor when it is turned off, sure it's not overheating...........yet.
when things get hotter they expand, when thens get colder they contract.
Even tho things inside the hard drive are only expanding by a very small amount, at one point that small amount could be just enough to crack something like the head and therefore cause faliure.
the heating/cooling cycles induce cyclic stress on the hd. the more cycles it goes through (and the more drastic the high/low points of the cycle) the more stress the hd experiences. your springs, precision gears, etc will really start to feal the wear after 1000's of extreme temperature cycles.