You do not need to have an approved route for an event when calendaring it, and you can decide to use a different  approved route at any time up until the event is held. (You might do that for safety reasons because of emergent weather conditions, or unanticipated road closures.)

You may associate an approved route with a non-team event when you submit the event, and when you submit results for that event. The associated route is displayed in the results for Rides Search. If the route is well-named, it can give the rider a good clue as to where the event will go. [12/2017]

 “Within or Through” Search – One reason to associate a route with an event is that the event will then show up when searching for rides within or through given states. Until you specify a route, the website can’t tell which states the event goes through. (It cannot infer anything from the event’s region.)  This is usually not important, but could be if the rider is interested in accumulating states for the American Explorer Award. [12/2017]