From CoMC/HoM Lesson 1. The question was... why would the Founders include the Lake in their designs for Hogwarts?
And Sierra Greingoth's answer was...
Quote:
Originally Posted by
Anna Banana 
"That lake can also be used for many different things--we can come and go, others can come and go...and we can learn about things like water plants and creatures," she said. It was also used to ship the first years to the castle, holding them up, so that the older kids could get in first and snag all the good seats. "So, I think it's been used for all those reasons in the past." Maybe a lot more, too. That was why they were here, though--to learn those reasons.
Emphasis added by me. Clearly it's to allow the older students to get the best seats at the start-of-term feast. Clearly, Sierra, clearly.
