Polyphon channels play more notes at a time. This operation reduces and removes overlapping notes so that each channel plays only single note at a time. Pedal MIDI controllers are removed, since they influence polyphony too.
Mono phon melody is used in some applications like phone melodies, search by melody (e.g. parsons code).
Hint:
Create a MIDI file from a polyphon MIDI song by removing all channels except melody channel and then use this operation to ensure that the melody channel will be monophon.