Why do radio stations start 6 minutes past the hour or half hour?

For ads, the hourly news update and for the syndicator to play there ads in Leo's case Premiere Radio Networks ;)

Many start 5-6 minutes past the hour when they broadcast through multiple stations. the reasoning is to get everything ready during their slot and also get in contact with their board operators.

It also allows them to account for commercials that radio stations add that are outside of the the ads that the show it's self uses.

For example if you listen to a popular radio show such as free talk live http://www.freetalklive.com/

if you tune into 1 station broadcasting it and during the commercial switch between a few, you will see that the different stations are playing different commercials and at their &PM eastern time slot, you will hear that not all radio stations end their commercials at 7PM exactly.

To add to this usually radio stations have to identify themselves every half hour (or hour I'm not sure). They have to say the call sign and their frequency, also where they broadcast as well. Here's an example I know by heart. "HIFS-FM, Cima 100.5, Santo Domingo, la Area Capital, Santiago y El region Cibaeño en la Republica Dominicana!" Then they'd explain their power in megahertz and so on. That would be the long form obviously, but I know in New York, all they do is say their call sign, name, and city of where they'd broadcast. They don't go into detail like the ones in D.R. do.