Jump travel is incredibly hazardous. Misjumps can see passengers spend thousands of years in jumpspace or have them "blown" off course by thousands of light years. During the early days of Terran space travel, when much of known space was still unexplored, there was little guarantee that the destination you were jumping to would have a source of fuel to allow one to jump back or a habitable planet to allow ships to resupply. Certainly there were a number of electronic mechanisms designed to detect suitable planets, but due to the fact that these relied on light to gather their information, they often showed only what a given system was like hundreds or thousands of years in the past.
It was to solve this problem that the Directorate, or perhaps a Terran polity pre-dating the Directorate, created the first astropsychics. By using the psychic potential of the human mind that had already been identified decades or even centuries earlier, astropsychics were able to create images of sections of unknown space as it currently was. Thus, they could identify whether or not a planet still possessed water or a gas giant had remained relatively stable, thus ensuring the exploration ships would be assured of a return trip, barring other misjump issues.
Even the most powerful astropsychics are not omniscient, and thus over 96% of the galaxy remains uncharted, and areas such as the Boom Worlds have only been recently discovered through the use of "chaining." Initially, this practice began when astropsychics would telepathically beam the portions of the galaxy they had mapped to the next astropsychic an a long line in order to prevent the psychics from collapsing from exhaustion. Eventually the Bureau's non-psychic board decided that a system by which these astropsychics would be mentally linked and kept in quasi-suspended animation would both be more cost effective and would return results more quickly. This technology eventually led to the creation of the polyminds used as astrogators in Directorate highliners, the massive ships used to make the incredible jumps necessary to reach the Boom Worlds and other fringe areas of the Directorate.
Due to the potential for mental degradation and insanity, Ultra-Terrans are almost never subjected to the extreme forms of torture, drug regimens, and cybernetic enhancement required to create an astropsychic. Instead the bureau relies on a steady stream of drifters, vagabonds, and convicts in order to fill their ranks. The governing board of the Bureau, and most of its top members, are not themselves psychic, though they often possess certain cybernetic enhancements designed to shield them from the psychic powers of the lower members.
The average astropsychic - those which have not been merged into a polymind or driven mad - also possesses a number of skills that make them capable assassins. The Bureau has been unable to figure out a way to only awake the psychic powers necessary for advanced astrogation, though it is unlikely that they desire to do so. The Bureau sees itself as chronically underfunded, and it is a well known secret that to increase its funds they often train their astropsychics to be hired killers. These are then leased out to various megacorps, noble houses, and merchant guilds to allow them to compete in the microwars permitted by the Steering Committee in order to make sure the existing structure is "the fittest." While this is technically illegal, the exalted status of the Bureau combined with the importance of their clients means they are likely never going to be prosecuted or reprimanded.