Angular guards, unclear statement in the docs

I'm trying to understand angular deeply, so i read the docs and it was very helpful.
now i'm studying the guards. and i read this statement in the docs.

The router checks the CanDeactivate and CanActivateChild guards first, from the deepest child route to the top. Then it checks the CanActivate guards from the top down to the deepest child route.

now i'm confused, why does angular perform it in this way?
is there any benefits of doing the checking from the deepest child to the top for CanDeactivate & CanActivateChild. and from top to the deepest child route for CanActivate?