In the golden age of radio, most big time radio shows took a seasonal hiatus, and the networks filled in with Summer Replacement shows, allowing big name radio stars to have a break, and letting up and coming stars break into show business.
As radio's influence grew in the 1930s and 1940s, big name and big money radio ruled the airwaves. Many radio programs centered around a special radio star such as Jack Benny, Fred Allen, Dinah Shore, Red Skelton, Bob Hope, George Burns and Gracie Allen. As their fame and influence grew, they requested special privileges and most - understandably - insisted on summer hiatus from their radio gigs.
Hope and Bing Crosby were also film stars, of course, and were able to leverage time away from their radio programs to make films. They created "Road to...(Singapore, Rio, Hong Kong, etc)" movie series while on break from their respective radio shows.
But others, like married couple Jim and Marian Jordan, who portrayed Fibber McGee and Molly on radio from 1935 until 1959, took a real break and used the summer break to spend time with their children.
Networks struggled trying to fill airtimes and retain loyal audiences because in the Golden Age of Radio there were no reruns. Radio shows were heard once and fresh material was used in every show week to week and day to day. Transcription technology was limited and recording radio shows was expensive. So, they turned to summer replacement series to hold their audiences.
In most cases, summer replacement shows were low budget and sometimes so bad sponsors even pulled funding mid-season, making producers scramble for more cash. Some of these replacement shows were dropped from the schedule when the original series returned, never to be heard again.
But others won over audiences and became radio hits of their own. Radio shows like The Aldrich Family, Boston Blackie, The Doris Day Show, Escape, Our Miss Brooks, Philip Marlowe, and Sam Spade were all summer replacement shows that won over audiences, producers, and advertisers to become a regularly scheduled program.
And despite the small budgets and low pay, newly rising stars were eager to win a regular spot on the radio. For this reason, many of the summer replacements were high quality shows. The now infamous Orson Welles production War of the Worlds was created after Mercury Theater was picked up by the network and added to the schedule.
(Jon Foulk is a radio historian who runs otrcat.com, the Old Time Radio Catalogue website, a cornucopia of radio shows.)