Last October, one of our Dynoswimmers, Judi Rich was asked, "What's the purpose of warm-down?"
So since we often have swimmers that come late to practice or leave early, it seems that we should clarify the importance of both the warm-up and the warm-down. To that question, Judi correctly replied that swimmers need to warm-up muscles and elevate heart rates during warm-up and relax muscles and bring down heart rates at cool down. Another Dynoswimmer, Glenn Partelow then mentioned that the warm-down helps relieve the lactic acid that builds up during high intensity workouts.
Judi found this article from USA Swimming that explains it all very well.