The Composer field is optional and is not enabled by default. Most stations do not use it. However, some stations that play a lot of jazz and/or classical music like to keep track a song's composer as well as its performer (which they then enter in the artist field).
It is possible to make the Composer field mandatory, in which case DJs cannot submit the playlist entry form with this field left blank.
If the Composer field is disabled (as it often is) then the composer of a classical piece is often stored in the artist field and the performers may be mentioned in Song note. See the Artist name help text for more details.