Line input transformers are used to protect audio line inputs in studio
and live equipment from picking up external noise, caused by,
for instance, ground loops or cable noise pickup.

Line input transformers should have a high signal level capability, high no-load impedance and high Common Mode Rejection Ratio, CMRR.

Even short cables connecting two audio devices may sometimes pick up noise such as radio signals from cellphones or other transmitters around the setup. Such signals are however of common mode character, and are efficiently blocked by the Faraday shields in the input transformer.
The degree of the capability of the transformer to block such
signals is expressed as the transformer’s CMRR.

Ground loop noise can be induced when there is a direct (galvanic) connection between the signal source and receiver, and there is a ground level potential between them. This can occur, for example, when devices are connected to different wall outlets. It can also occur when lighting dimmers are connected to the same power distribution system as audio equipment, as dimmers have a tendency to make the electrical ground reference very noisy. But as the primary and secondary windings in an input transformer are galvanically isolated,
line input transformers eliminate ground loops.

A line input transformer will also take care of any balance-unbalance or unbalanced-balanced conversion needed if both balanced
and unbalanced equipment are used.