Until 1921, you had to be at the ballpark to follow a baseball game as it happened.  

But on August 5, 1921, Pittsburgh's radio station KDKA provided its listeners with the first-ever broadcast of a major league game. Harold Arlin, a twenty-five year old nighttime studio announcer, described the action in the Pirates 8-5 victory over the Phillies at Forbes Field, becomes baseball's first play-by-play announcer. Sitting in a ground-level box seat and using a converted telephone as his microphone, he calls the full nine innings.

Arlin said later that the broadcast was an experiment, and that most of the staff thought baseball would never make it on radio - "too boring" was the general opinion. But the experiment was a smashing success, and quickly spread across America. At first, the owners were reluctant to broadcast their home games, as they were afraid the fans would just stay home and listen to the games for free (games in New York, the game's biggest market, weren't broadcast on a regular basis until 1938). However, it  turned out that interest in the game and the players increased dramatically, and attendance jumped as a result.

.