Are you familiar with this policy? This instructs the employees to make sure that all of their computers are locked especially when they have to leave their desks for a certain period of time. There is always a possibility that there will be prying eyes.
There will be people who will look into the computer so it is best that the laptop/computer will be protected. At the end of the day, laptops may have to be kept in safe places so that it will not be accessed by anyone. Do remember that there are some people who can always hack the password.