It is usually, sadly, a thinly disguised ploy to make money, with creativity taking a back seat. The job of the Big Event Crossover is to get you to buy the extra books as well as some books that you normally don't read, in order to follow the story. The hope is that you will stick with some of them after the Event is over. Now that comics cost at least $2.99 apiece and usually closer to $3.99, it can turn out to be a substantial chunk of money for the dedicated reader.
When I was a kid, DC dealt with its then-30-year history by coming up with the Multiverse. Those stories from the 1940's, when Superman and Batman were just starting out? It turned out they were taking place on another Earth, Earth-2. This was why "our" Superman and Batman weren't 60-year-old men still trying to fight crime. This turned out to be rather creative on DC's part. When they picked up another company's characters, if they didn't want to incorporate those characters into the main DC Universe (aka "Earth-1") they gave them their own Earth. If you were a Charlton character like Captain Atom or The Question, well, welcome to Earth-C. Captain Marvel and the rest of the Shazam! gang? Earth-S.
My favorite Earth, though, was Earth-3 -- the Earth where Evil always triumphed over Good. There was still a Justice League, but it was called the Crime Syndicate of Amerika and was made up of evil versions of heroes like Superman, Batman, Wonder Woman and Green Lantern.