There are conditions that may make an average person unable to perform the movement, like getting a severe cold or an eye infection that makes an eye water. The

There are conditions that may make an average person unable to perform the movement, like getting a severe cold or an eye infection that makes an eye water. They may still want to use their phones with minimum inconvenience, instead of resorting to entering a passcode. It may not be possible for some people to configure it well either, and may probably result in eye movements that others may consider weird (imagine rolling your eyes in front of your boss because you wanted to unlock your phone).