Sailboats - why lifelines and not solid rails?

[SteveG]

Ok, I was at the Boston Boat Show today, and I asked this question of a bunch of people working the sailboats. None of them had an answer.

Why do sailboats not have solid rails on the sides, but rather wire "lifelines" as they are called?

Thanks

P.S. For the record, my buddy likes to sail, so we were looking for him.