By changing the number of wire turns in each winding, transformers can be manufactured to have specific input and output impedances. The ratio between the input and output impedances provides a gain or loss of signal level as the signal passes through the transformer. Transformers are bidirectional so that an input winding can become the output winding and an output can become an input. Because of a transformer's bidirectional nature, it can provide a gain in signal level when used in one direction or a loss when used in reverse. Transformers can be manufactured with multiple primary or secondary windings.
Different impedances cause the signal level to change as it goes through the transformer.
A transformer with multiple taps provides access to multiple impedances and to different signal gains or losses. Many microphones have step up transformers at their output. For example, inside of every SM57 and SM58 microphone is a transformer that steps up the signal level and impedance before it exits the microphone. As the impedance is identical for the primary and secondary, the signal level does not change. Also, since the primary and secondary are insulated from each other, a unity transformer will electrically isolate different pieces of equipment.