Prior to 1944, it was not common for a film to return to theaters once its initial run was over. Regardless of how beloved or successful a movie became, audiences weren't guaranteed to ever see it again once its original release was finished.
In 1944, however, Walt Disney Productions decided to test out a reissue of Snow White and the Seven Dwarfs. In the seven years that had passed since the original release, a new generation of children had grown up never having seen the film.
The reissue was a huge success, starting a tradition that continues until today, with DVD and Blu-ray reissues of Disney's biggest hits.