Crossover events are really just an intensification of the shared universe that exists in most mainstream superhero comic books these days.
One of the earliest mega-crossover events, DC's 1985 Crisis on Infinite Earths, was done as a way to clean-up the mess that DC continuity had become. Of course, the relative simplicity that resulted didn't last all that long. The company-wide reboot that was done after the event wasn't even rolled out all at once. So events don't clean house, they make things more complicated. Many creators have been quoted as dreading crossover events because they're a pain in the neck to work with, and can even interrupt ongoing storylines. However, like shared universes, companies seem to demand periodic crossover events.
Superheroes appearing in each others' titles was hardly new even in the days of Crisis on Infinite Earths. In the 1960's, DC and Marvel characters were appearing in each others' titles - and it wasn't even a new idea at the time. Characters from earlier on, such as Alan Scott and Captain America and Namor, have since been folded into mainstream continuity. The reason often given by creators is that this cross-pollination results in a richer shared world for the characters to inhabit.
However, the real-world reason for a shared universe is that, if you see Spider-Man appear in your Fantastic Four issue, you may go pick up Spider-Man. (A similar logic prompted Marvel to add footnotes to events referred to by characters, in the hopes of encouraging back issue sales.) Crossovers are just a more intense way of encouraging readers to pick up other titles. And this way, fully-developed characters could then be used in team books, like the Justice League of America and the Avengers.