I guess it may have been a cheap and easy way of the developers ensuring every town had a pond?
this is probably why. like, you /have/ to have at least one pond bc some fish are only pond fish.
the retail one was just a way to guarantee at least one pond to fish in.
My personal theory (also goes for the pond outside of the museum) is so a player can go outside and dump any fishies you can't donate or that aren't worth selling.
Maybe. I dunno.