The Original universe of each character could stand on their own as good/great stories, so why make alternate universes with different stories that couldn't cross (in most cases) with the same character of the other universe?
I think you're thinking about this from the wrong direction. Each character doesn't start out in their own universe: they all start out in the same universe! So you've got a single universe that contains the X-Men, Silver Surfer, Captain America, Spider-Man, Iron Man, etc. This means that when you're reading a Spider-Man comic, Tony Stark is walking around somewhere in that world, doing whatever he's doing over in the Iron Man comic. You can think of individual comics as windows into that single universe, shared by every Marvel character.
So, why create a new universe? Well, after 50 years of these stories, the universe can be pretty confusing, not to mention outdated. Maybe somebody wants to tell a new story without all of that baggage. Maybe they want to tell Spider-Man's origin story, but set it in the present day instead of 1960s. Or maybe you want to tell a story about all of the heroes turning into zombies.
The problem with that is, you can't just ignore the stories from the previous universe, otherwise people feel like you're invalidating the characters. So the solution is easy: you simply say that your story takes place in a different universe. That way you can tell your new stories, and the old stories will still be "true".
That's the "why", but as for the "when", check out this Marvel wikia or this wikipedia article for a list of universes within Marvel, as well as the dates they appeared.