Radio Use Only is a fascinating site devoted to those releases record hounds often come across in thrift stores and used-record shops' bargain bins - albums released by radio stations.
The practice "...started out as promotional items to spotlight a certain disc jockey or a particular show on the station. Other early-known radio compilations would feature various popular songs, of the time, in a pre-packaged set. Some of these would also include audio samples of station disc jockeys introducing songs...Starting in the early 1970’s, radio stations began sponsoring albums featuring local, unsigned performers."
The blog has audio samples. I particularly like this "groovy" and "happening" Dallas-area radio station jingle:
KXXK jingle (circa 1968-1971)
Which reminds me of this contemporaneous (and seasonal) jingle from the Free Design:
The Now Sounds of Christmas