Hi Ryo Okada!

There are two questions there I think:

Why do you need to dock navigation?
You need to doc your navigation because if someone has scrolled down too far, they won’t be able to navigate to a different page without scrolling up for 10 minutes.

Why dock to the top?
It doesn’t really matter where you dock your navigation, it is just that most sites dock it at the top and that is what users are most comfortable with.

Hope that helps!