What is the use of dual input, unbalanced output differential amplifier in an operational amplifier?
In most applications the unbalanced output works okay and is simpler and cheaper than balanced outputs, which are normally required only in rather unusual circumstances. E.g.:

(a) when driving high-frequency twisted-pair transmission lines. (The electromagnetic common- and differential- eigenmodes propagate at different velocities so a differential driver is needed to launch a 'pure' differential mode.)

(b) in high-precision applications, where the unbalanced ground currents and finite conductance of the ground paths can cause error signals.