In his current state, the Emperor of Man on his Golden Throne generating the psychic beacon called the Astronomican which makes warp travel possible.
The Astronomican is a psychic beacon which the Navigators utilise to pilot the spaceships of the Imperium through the otherwise unnavigable chaos of Warp-space. 
But what about the time of the Great Crusade?
Thousand of worlds were conquered throughout the galaxy in a period of 200 years, so mankind MUST have had the means to warp travel and with the Emperor moving around he could not have generated a fixed psychic beam that would make navigating through the warp possible, even if he would be strong enough to do that without the Golden Throne.
Is there a canon explanation for how mankind traveled through the warp during the Great Crusade?
Sources:  http://wh40k.lexicanum.com/wiki/Astronomican