Later, they became almost a necessity for a 154-game schedule because teams traveled by between cities and needed a lot of off-days in their schedule to get to places like Detroit, Chicago and St. Louis. That all changed with air travel, even with teams spread all over the country.
Today, a lot of things like the designated hitter and doubleheaders are governed by the Major League Players Association agreements. Players need to hear about it if things are to change back.