A hundred years ago, long before there were radio stations that played music, a hit song was determined by the number of copies of sheet music it sold. There were many ways to promote a new song.
One way was to send a representative from the publishing company around to well-known singers of the day to try to convince them to include a new song as part of their act on the vaudeville circuit or in music halls. If the singer agreed, the publishing company would often plant people in the audience who would jump up and applaud wildly when the song was performed, demanding an encore.
Seeing this, other members of the audience might think “If that guy thinks the song is good, then maybe it is” and stand up and cheer themselves. This was done night after night and in city after city until the song was popular enough for people to buy sheet music so they could play it at home. It was exhausting, but it worked.