This is also, of course, why you are asked to adopt the brace position in the event of a crash. I don't think there's much evidence that it makes much difference to survivability, but it does get the heads below seat level so less of them are chopped off by bits of flying metal, thereby making the identification of remains more straightforward.
Although personally I have never understood why this is important - if you know who was on the flight then you know who is dead - putting the pieces back together won't make them come alive again!
