Do you want to do it again and again? Just to show you can. Or show it to all your friends.
Do it again to make sure you remember how to do it just right?
Experiment a little, adding some new elements to your achievement?
Sometimes you just like to do the same thing again...to keep your skills honed.
You never know when that skill may become useful.
(I watched this boy repeatedly build his blocks in a line and knock them down. Day after day. Around what the other kids were doing. Sometimes he would build something else. But he always went back to this familiar--and really cool--construction.)