What I've been reading on pump design is the type of impeller is specifically chosen for the type of fluid. More open and simple impeller like this perform better and more reliably in conditions where you might have solids from a pond or algae in an aquarium where a very dense type of impeller would have trouble plugging.

Just like radiators, we're getting the technology from a hand me down industry that wasn't water cooling computers.. I don't think any of our pumps we use today were only designed for water cooling PC's.