Ah, the lake. By now every student present had some experience with it, even if it was just coming over it by boat the first night of term. Keefer liked to take walks around it in the fall and spring, so it was one of his favorite places on the grounds. I wonder how much the lake has changed in the thousand years Hogwarts has been here. Because in that much time, natural processes were bound to shape it in some way.

Keefer listened as Scabior pontificated talked. Upon hearing the questions, Keefer raised both hands into the air. Yes, both hands. Don't judge.

"Professor, it seems to me that the Founders would have built a castle by the lake for the same reason as many ancient or medieval civilizations sprung up by major bodies of water. They didn't exactly have plumbing back then." He grimaced at the thought. "Having such a large body of water this close would allow the castle to house hundreds of students without fear of not having water. The Aguamenti charm would be impractical for such a large amount of people." He paused and thought for a second. "I suppose there's also defense to consider. Any Muggles or enemy wizards would have a hard time approaching the grounds via water without being noticed. So I guess that leaves less terrain to keep track of."

As for the other half of his question... "I suppose they used the lake for much the same things we do today. Beauty, studying, recreation. Maybe they even used it to power mechanisms like Muggles used to, though I bet they could think of a magical means for that. Obviously the water supply aspect I already mentioned." Yup. That about summed it up.