Do you mean in general? There are two reasons given in this thread: medical research and pet (snake) food.
I expect I don't need to explain why it's best to kill the mice humanely for medical research which involve dissection of some sort.
For snake food, it's best to feed mice that have been euthanized. Feeding live mice is cruel to the mice, who experience terror right before death, and there is also the possibility that a struggling mice could injure the snake.
