It's a pedestrian/bicycle safety issue. In the US, highway guard rails (or guide rails, depending on an agency's terminology) line up with the bridge parapet or the bridge rail on roadways that prohibit pedestrians & bicycles. These are generally high speed roadways such as interstates. Sometimes these roadways have trails for pedestrians & bicycles. In which case the railing is at the front of the sidewalk/walkway, in line with the curb. Sometimes, a bridge doesn't have a raised sidewalk/walkway therefore requiring a barrier between pedestrians and vehicles.

If the bridge has a curb, then the face of guardrail must match the face of the curb, per Roadside Design Guide.