Alternate universes allow readers to have a version of Batman who finally got rid of many of his scars of the past and fell in love and got married and had a kid.
It allows Peter and MJ to grow up and have a daughter and get put through the rigours of raising a superhero teenager.
It allows someone to write a Superman/Batman/Wonder Woman story set in WW II.
It allows someone to play a game of What If and ask 'What would happen if Booster Gold had missed when he originally came to the past and instead of meeting and teaming up with Ted Kord he actually met and teamed up with Ted's predessesor, Dan Garrett?' Oooh, now I want to write that story. And I might even want to write it using the radio show version of Dan Garrett in which he was a police officer who gained super powers through a 'miracle drug' rather than the archaeologist version who gained his powers from the scarab.... FANFIC HO!
But anyway, you get the idea.
If people weren't so hung up on canon. Despite the arguements among Doctor Who fans they really have the best of all possible worlds as the BBC, the writers and the producers have basically said "If you want to believe it's canon then it's canon for you." So all the novelizations could be canon. All the comic book stories could be canon, all the audio stories could be canon. Or some of them could be, or some of them could not be. It all depends on what you desire.
If you want 1960's Spider-Man to be canon... then it can be. If you want May Parker to be canon then she is. etc.