For movie goers sound helps create the overall presentation of a film but when it comes to radio, sound effects help assist listeners and "the theater of the mind". Otherwise radio listeners would just hear actors reading lines from a script which without sound effects would become pretty dull and boring.
In the summer of 1972 and prior to attending the RCA Television Studio School in Times Square that fall, myself and family took the NBC Studio Tour in New York City. Some forty plus years later I don't remember too much of the tour except for the group we were in standing in a darkened hallway with curtains about waist level and up toward the ceiling. When the curtains opened we realized we were looking through darkened windows behind the studio audience looking down at the studio stage of the Johnny Carson Show.
The other thing, and which stands out the most was at the beginning of the tour we were taken into an example of an old radio studio complete with showing us how sound effects were created for live radio programs. Of course during the Golden Age of Radio all programs aired live, gaffs and all, often with a studio audience either in the same studio or on the other side of windows with mics overhead to capture audience reaction. What amazed me and in a way spoiled the illusion as showing how magicians do their tricks, was how the simplest of things were used to create sound effects totally different from what the item(s) actually were. I found this webpage that goes into the detail of how sound effects evolved in radio, the best part of the page is you can click on several sound effects that had been used in those days.
Below is an excellent short film showing radio sound effects artists creating the all important something extra to complete the entertainment of sitting around the radio listening to a show.