Pond Lining

This weekend, the guys in my family are working on my grandpa's yard. They're re-doing the ponds that PawPaw put in several years ago. Their main goal for the day was to put in a pond liner. He had drained the pond because it was growing too much algae and such. Why was there overgrowth? Because there weren't any fish in it. I don't know that he thought about that. He just thought, we need to get rid of that algae. But the algae really is part of a natural system. If he had thought to really create an ecosystem out of his ponds, it might practically take care of itself.

But instead, they put down a liner and spent the whole getting sore and sunburnt. If I understand correctly, the liner is intended to not allow the soil beneath it to produce anything that would interfere with the pond. I'm trying to figure out if that's an "ethical" thing. He is intentionally smothering the soil for aesthetic purposes. I'm not sure how the pond will stay clean, even with a liner...

The whole ordeal doesn't make much sense to me. It seems like a whole lot of work for little reason.