great Question. DNA is a really long molecule and although it is relatively persistent (it stays around a long time in the right conditions) it does start to deteriorate at the point of death. bits can sheer off and the molecule breaks apart. making it much harder (sometimes impossible) for the bioimformatitions to piece it back together. by flash freezing it to a really cold tempreture using liquid nitrogen and dry ice, the molecule is preserved in a more pristine condition making it easier to read in the future.