They explained it in the May 15, 1955 Watchtower (pages 305 and 308):
Page 305…
16 From the many recorded experiences we next observe that the official “Word” transmits the message either by means of an angelic messenger who appears face to face or by the use of God’s holy spirit to inspire the nextagent in the line of communication . On special occasions even the Word himself might appear in a face-to-face interview with the next agent. Who, generally, is the next agent? For supernatural communications that reach the earth the agent (generally a human, although once the ass of Balaam was used) is referred to Biblically a prophet if he is a male servant of God or a prophetess if a woman. The prophet, in turn, speaks out the communication as God’s spokesman on earth. He publishes it to the many whom God intends to hear it if other than a single person is being specifically addressed. Theprophet then becomes the immediate earthly channel through which the waters of lifegiving truth are conveyed to earth’s thirsting residents. Thus Jehovah’s earthly channel of communication is identified. The earthly channel is either a prophet or a collective prophetlike organization .
Page 308…
TRUE Christianity as a revealed religion relies fully upon all the progressive revelations of Jehovah’s sacred pronouncements communicated to the earth from the time of Adam to that of the apostle John. Additionally we see how God has provided his Christian servants from Pentecost, A.D. 33, forward with a reliable channel of guidance for the understanding of these amazing revelations of his will and purposes. From the prophetic shadows to the actual realities we observe that this God-provided channel for Christians is the collective congregation of anointed ones who serve as a prophetlike organization under the leadership of its communicating head, Christ Jesus . –Eph 5:23