You’re free to go where the road takes you, without worrying about the odometer. Unlimited mileage is ideal for leisure trips, where it’ll be nearly impossible to determine exactly where you’re going for the whole trip. Part of a vacation is about being spontaneous and trying out a new area, and being chained to a mileage limit can be frustrating. This is more of a cost up-front, but if you can reasonably see yourself using over the number of miles included with your rental, it’s worth getting. If you want to head out for a late-night snack, check out a few local shops that weren’t on your agenda, etc, you shouldn’t have restrictions impeding your vacation.