As others have mentioned, there's a social aspect to cycling where it's simply fun to get out and ride whatever distance with like-minded others.

If you're strictly concerned with just biking from A to B, or B back to A, then I do it for the fitness and endurance, knowing that I can indeed reliably and consistently bike from A to B where the distance between the two place is a hundred miles or more, not just down to the office or the coffee shop and back.

And for what it's worth, a 100-mile ride is called a "century," not a centurion.