The single-ended or differential configuration is chosed by the designer, depending on application.
Single-ended - ground is the return path for the signal.
Differential - ground is not used as a reference, so the ground coupled noise is eliminated.
Differential - because the signal subtraction, any common mode signal (same amplitude and phase with reference to gropund on both inputs) is eliminated - egg parasitic signals present on long balanced cables.
To convert between single ended <-> differential, u can use a transformer or coupler.