To indicate when and at what pitch and tempo the singing should start, and to provide any necessary rhythmic and pitch support to keep everyone together and stop them drifting flat, and then to indicate when to stop singing, presumably.
I take it to mean “no music for its own sake” but only where it is essential to allow singing to occur. No instrumental pieces and probably no flourishes and “frills” such as solo reed stop descants superimposed on the final verse, type of thing - things that are nice but not necessary to sustain singing. If the congregation couldn’t sing without it, do it; if they could, don’t, sort of thing. For some things that will be a more-or-less normal accompaniment, for others it might just be a soft starting pitch.