> The employee / manager relationship should be one in which either party can feel free to just raise any concern the moment that it comes up.

This is true in an ideal world, but not everyone is comfortable being direct with superiors. Regular 1-1s establish a baseline and a foundation that can be built on. Plenty of people will delay raising concern until damage has already been done. Regular, private communication lets the manager get ahead of these issues and handle morale problems before they surface.