1 reason can be that when you have a event you can attach it to several categories and when there is a change you only have to modify 1 page.
Have to say that in my case i also don't use it and am not going to link multiple cats to 1 event. Maybe i'll add an option in my custom test version where you can simple copy the event to several categories. So that in case that i do want to have it linked in several categories it can be done.
Another reason for multicat might be the registering/attending part. When attached to multiple cats there is 1 eventpage.
Ah well there is a tag system in joomla and multiple tags can be added to 1 event.