In addition to this invaluable beacon among the stars, the Imperium would not have been made possible without the creation and continuation of the Astra Telepathica. This special corps of inter-stellar communicators was created by the Emperor during the final months of the conquest of Terra, as the Emperor had foreseen their absolute future need.
For the most part the Emperor did not favour the use of psychic talents in others, as a psyker himself, albeit one of unprecedented power and control, he was well aware of the dangers inherent with contact with the Warp. Furthermore the Age of Strife on Old Earth had its own 'witch-kings' and possessed psykers to attest to those dangers, some of whom the Emperor had personally slain. However, he was able to identify those strong enough and so with some prudence certain psykers were employed in a variety of special roles in the Imperium, and the astropaths were to be such and exception.
The paradox of the psyker was that while a pressing danger to humanity, their presence in one form or another was utterly vital and inherent to the pursuit of the Great Crusade. On a practical level this was embodied by the Emperor and, most widely and vitally, by the bloodlines of the Navigators who facilitate long-range Warp travel and the Astropaths that make interstellar communication possible.
The rarest and most powerful found themselves in the employ of other, more secret, Imperial organisations whose shroud of mystery still holds fast.