Most shoulders are clearly marked with a line.

Solid white line

This line is made for emergency stopping, standing  and parking. This is usually slightly narrower than the full traffic lane. This is used primarily for emegency or breakdown. Emergency vehicles can bypass the traffic congestion. This can also be used to allow general traffic during high volume traffic. This can also be used as sidewalks, pedestrians and cyclists to walk or ride. This can also help in the structural integrity of the road.