This feature is supposed to adapt to changes in your face over time. You grow facial hair, change your glasses, wear a mask, etc, but it has to work anyway.

How do they do that? Naturally by getting as much data about your face as possible. It would make sense to update every single time you unlock the device.

I’m not sure why every five seconds, but maybe because you might change your appearance in that time. (put a hat on or something)