A few things. First off, it's practically the ultimate baby thing to do, just to sit there, let go, and relieve yourself just about anywhere.
Secondly, convenience. I'm a busy guy. I work a lot and I'm in class a lot. When I have free time at home, I don't like to waste it, and often I'll diaper up while home. Even if it saves just a few minutes whenever I mess, it still saves time.
Third, it's kind of a turn on for me, wetting and messing my diaper.
I could probably go on and on, but a lot of what makes it enjoyable for me is within these few reasons.