The Listener is the only member of the Dark Brotherhood who can hear the Night Mother. As such, he or she is effectively the mortal leader of the Brotherhood and bears the responsibility to deliver the full text of new contracts to the membership for assignment. As the living representative of Sithis and the Night Mother, the Listener's word is law.
The manner of selection of a new Listener is a somewhat arcane process, as only the Night Mother herself selects a new Listener. Without a Listener, the Brotherhood can find itself running without clear direction as it becomes more difficult to acquire contracts. Once the Night Mother does select a new Listener and that individual becomes known to the Brotherhood, the traditional system of leadership and execution returns to the norm.
It is this relationship between the Listener and the Night Mother that has created much of the mystique around the Dark Brotherhood, granting its uncanny ability to acquire contracts and contact with customers via the strange and powerful Black Sacrament.