...some dispatchers around here..
To the original poster asking the question, SpUd is correct about MX driving things. Aircraft routings are usually designed so that the aircraft ends the flying day at a MX base every so many days for routine inspections and checks. When it departs the MX base the next morning, it could end up anywhere on the map, thus allowing other aircraft in the fleet to be cycled in on the same pattern.
What gets fun is when aircraft have problems during the day, and have to be either fixed where they are, or re-routed into MX that night for fixes. Some profblems can be deferred, say, like a windshield wiper. You can go with it INOPerative as long as there's no rain at the takeoff or landing airports, and if there's a chance there's gonna be, we dispatch types will route the aircraft from the offending weather and send it someplace where the weather is good. THis sometimes means unscheduled plane changes for passenger and crew alike, but we're looking at the airline as a whole (keeping it safe, legal, and on-time) and keeping passenger inconvenience to the minimum possible.
ALL views, opinions expressed are mine ONLY and are NOT representative of those shared by Southwest Airlines Co.