Before the Silver Age of comics, comic books tended to have a loose interpretation of continuity. If previously published comics fit into a story that a creative team was currently telling, that was awesome. If it didn't, someone sat around trying to figure out why and then they published something in a comics fanzine somewhere.
But, as of 1961, there were two Flashes, and they lived on Earth-1 and Earth-2. Weirdly the heroes who were around first were relegated to Earth-2. But for the first time there was an in story rationale for the then current Flash, Barry Allen, and the original, Golden Age Flash, Jay Garrick, to more or less coexist. And if both of these Flashes lived in the same universe that meant that the two characters could team up together. Fans of the Jay Garrick Flash of the 40s could read new stories about the character they loved, and hopefully find out what he had been doing with himself in the interim.