You get to customize the exterior of your house for free - roof, siding, door, mailbox. But it's pretty inexpensive even without paying off the loan. I think something like 5k bells?
A huge pressure gets lifted off your shoulders because you know you're not in debt anymore and this little voice inside your head isn't reminding you about it every time you walk past resident services... also, the completionist inside all of us stops feeling restless.