What's the point of board-internal differential pairs?
A trace on a board may also be subject to radiated signals coming from elsewhere. A differential pair will likely pick up less cross talk from such sources.
A board may be part of a system that is subject to regulations that limit the radiation it is allowed to produce. A differential pair will likely produce less emissions.
A differential receiver will be less impacted by the receiver's reference level shifting due to power supply noise or ground voltage variation.