This comic, "Flash of Two Worlds," from 1961 brought the idea of the "multiverse" as we know it to comic books when Jay Garrick, the Golden Age Flash, teamed up with Barry Allen, the Silver Age Flash and the concepts of "Earth-One" and "Earth-Two" were introduced.

Yeah from that point on DC treated the Golden Age/JSA as a separate universe from the Silver Age/JLA, and continued on with multiple earths to explain continuity gaps. Then of course they course corrected with Crisis on Infinite Earths. Then they did it again. And again. And again. Meanwhile Marvel had the concept of alternate earths (like Squadron Supreme) but mostly just kept general continuity rolling (like Tony Stark becoming Iron Man during the Vietnam War) and fudged it.

The multiverse was cool...the first 100 times I read it. It's become a cheap gimmick and storytelling crutch for the comics to introduce alternate versions of existing characters without upsetting fans. Now it's quickly becoming the same for movies.

For movies it's just fan service, pulling together disparate cinematic universes that were never meant to be in continuity. Any kind of fan service can be a crutch but I don't see it as a storytelling crutch the same way it is in the comics, where everything needs an explanation because of tight continuity, despite the fact that the comics span decades and hundreds of different writers.

Because they had a plan from the outset and it turned out great. DC saw that and instead of creating their own plan they tried to emulate what Marvel did and catch up to what they were doing at the time. They failed miserably.