That heading sensor is to determine direction since GPS cannot calculate direction without moving. Same as any other heading sensor.
Its function is for the jogging feature. Jog left, jog right, etc.
Humminbird Helix 12 CHIRP SI, 1198c SI, 1198c SI, ONIX 10 SI NT, Helix 7 CHIRP SI GPS G2, SOLIX 12 SI, 360, RC-1