Because when renting, one never builds up any equity. 
Good starter list, Brettski. I submit the following:
-Relaxation (just by being there)
-Reduce local insect population (a stocked pond is a hole in the bugs)
-(You didn't exactly in my mind include) Growing large or interesting fish
-Non-powered water sports (canoeing, rowing)
-Wildlife attractor (for aesthetics or hunting)
-Erosion control