This circuit demonstrates one of the main advantages of the differential amplifier, noise rejection. Assume we have a balanced input in the amplifier (ie one input is our signal and the other one is a copy of the signal shifted by 180 degrees) , due to proximity the two signals will pick similar noise . Then since the output of the amplifier is differential, the two noise components will cancel out , while the output signal will be added with its copy ( V1-(V1)=2V1) resulting in a amplified noise free signal.
