Smokin' and all pretty well answered the question,but there are a couple of other "reasonings" behind the "let set out for awhile".
None probably apply to home Cookshacks.
If you are cooking on a traditional log/charcoal burner,tossing maybe 100 lbs of 34� meat in the cooker is a heck of a heat sink.
It can greatly increase times to get to temp and to stabilize there.
Another new thing in the last couple of years,is that the government lets producers hold chicken at 28�, and still call them "fresh".
The producers are bringing much younger birds to "market" size/maturity and the bones don't have time to mature and seal,like they used to.
There can be some red/blood seepage from the bone,even in well cooked chicken.
Bloody Chicken There is some feeling that when fast cooking chicken,letting it rest out 30 mins"may" reduce this problem.
That being said,on my cookers,I go straight from the refrig to the cooker.