My farm, Mamushi Nature Farm, in Franklin, Tennessee, has several wildlife ponds. They are low maintenance bodies of water, self-contained ecosystems, in and of themselves. Organisms, both plants and animals, micro-organisms even, dwell within the wildlife pond, ecosystem.
I keep the wildlife ponds for spiritual reasons and for the "beauty" of them. Fish live in my wildlife ponds. I have nishikigoi (carp), some that can be eaten and some that are sold for $30.00 each once they grow to a critical size.
Therefore, one reason one may want to keep wildlife ponds is for supplemental income or for fish to eat. Watercress can also be raised in your garden in your wildlife pond. Lastly, you pond provides drinking water for wildlife.