| It helps to mute the unplayed strings, especially if you play chords or slap on a 5 or 6-string. Nevertheless, being able to mute unplayed strings with your right and left hand is a basic skill every bassist should have (and I am sure most that use the band-thing can well mute without it too)
The hair band is only an additional help, it's convenient depending on what you play. | 
