Why did the development of time zones make train schedules more reliable?

Before time zones, towns just used the sun and the point at which the sun was highest in the sky to determine noon and time in general. Since this isn't the exact same for all areas in the United States, the train schedules were always off. So, when railroad companies developed the time zones we use today, it made the time the same for vast areas so that everyone was going on the same exact time.