Prior to Crisis on Infinite Earths, DC was notorious for its continuity problems. No character's backstory, within the comic books, was entirely self-consistent and reliable. For example, Superman originally couldn't fly (he could instead leap over an eighth of a mile), and his powers came from having evolved on a planet with stronger gravity than Earth's. Over time, he became able to fly, his powers were explained as coming from the sun, and a more complex backstory (the now-familiar 'last survivor of Krypton' origin story) was invented. Later it was altered to include his exploits as Superboy. It was altered further to include Supergirl, the bottled city of Kandor, and other survivors of Krypton, further watering down the original idea of Superman having been the sole Kryptonian to survive the destruction of his world. There was also an issue of character aging; for instance, Batman, an Earth-born human being without super powers, retained his youth and vitality well into the 1960s despite having been an active hero during World War II, and his sidekick Robin never seemed to age beyond adolescence in over 30 years.
These issues were addressed during the Silver Age by DC creating parallel worlds in a multiverse: Earth-One was the contemporary DC Universe, which had been depicted since the advent of the Silver Age; Earth-Two was the parallel world where the Golden Age events took place, and where the heroes who were active during that period had aged more or less realistically since that time; Earth-Three was an "opposite" world where heroes were villains, and historical events happened the reverse of how they did in real life (such as, for instance, President John Wilkes Booth being assassinated by a rebel named Abraham Lincoln); Earth Prime was ostensibly the "real world," used to explain how real-life DC staffers (such as Julius Schwartz) could occasionally appear in comics stories; and so forth. If something happened outside current continuity (such as the so-called 'imaginary stories' that were a staple of DC's Silver Age publications), it was explained away as happening on a parallel world.
Some have said that, over the years as new readers were introduced to the DC Universe, the 'multiverse' theory — with its attendant multiple versions of Superman, Batman, Wonder Woman, et al — served to confuse those who did not have a working knowledge of DC's history. The editorial objective of Crisis on Infinite Earths was to streamline all of these parallel worlds into a single, consistent backstory, and thus hopefully make the DC Universe more approachable to new readers. It was also to free the company's writers from the baggage of 50 years of (dis)continuity.
The series was highly successful from a marketing standpoint, generating renewed interest in the company's books, enticing readers with the clichéd – but in this case accurate – tagline that "the DC Universe will never be the same." The story itself was rooted firmly in the cliché of "superheroes battle to save the world", but its unprecedented scope and its great attention to both drama and detail satisfied readers with its story. Along with Alan Moore's Watchmen and Frank Miller's Batman: The Dark Knight Returns, it contributed to the commercial and creative revitalization of DC Comics, which had been dominated in the market by rival publisher Marvel Comics throughout the 1960's, 1970's, and early 1980's.