Radio dominated the first half of the 20th century but the advent of affordable TV sets in the 1950s seemed to spell doom for the format. Yet, radio endured. Radio was able to create a distinct image for itself, separate from TV. I have own theories as to why (cars, transistor radios and the music industry) but nothing conclusive.
1950s were before my time but later in the same century, FM radio was what we had instead of internet music and streaming. You could call radio stations and request particular songs, or sometimes they would announce that they would play some new album nonstop starting at 7pm, so you would be ready with your cassette deck. Or in general, they’d just play music all day, so you’d always have something to listen to. That’s sort of what people do now anyway, except it’s whatever the youtube algorithm is promoting instead of what the record labels were paying the radio stations to play. TV was never a 24/7 music medium like that. I guess MTV was a thing for a while but that was still different.
AM radio on the other hand was mostly for news and sports updates, particularly in cars. So the most valuable advertising slots were during “drive time”, i.e. commute hours.