Listening to a syndicated show today, a caller complained about all the unproductive 'do gooder' ads he was hearing. This led to the host calling in his producer, who explained something I found interesting:

"We have spaces in our feed for the local station to use for their commercials. We put 'do gooder' ads in these spaces. If you are hearing them, that means there are ad spots that the local station has not sold. A smart business would see these 'do gooder' ads as time available for him to get HIS ad on the radio, and contact the local station to do so."

Think about this the next time you hear an ad to, say, 'save the whales from flatulence.' Wouldn't you rather hear "hey everyone, hire me!" ?

I know the other thing you will hear in unsold ad spots are those canned routines (like Rush's "spatula city", stolen from the film UHF).
I may be dating the last time I listened to him but that is what they do. They put some throwaway audio in the spot so it won't trigger the dead air alarm and the local station is free to try to sell that time. The more junk you hear, the less time they are selling.