On the odd occasion, I like to delve in to the Warhammer 40,000 background materials in a bit more depth. Today's topic for discussion is the (alleged) complicity by the Emperor in the Primarch's scattering during gestation.
In many of the publications we hear a tale that is somewhat paradoxical. If the Emperor is reasonably powerful and prescient, then he would have (a) seen the possibility of the Chaos Powers gathering their might to try to scatter his primarch creations and (b) had the power (and been in the right place at the right time) to do something about it. Indeed, in the Black Library novelisations of the Horus Heresy, it is hinted that the Emperor allowed or even permitted the scattering to occur.
If the Emperor was complicit in this, then there must have been something for him to gain. Generally this is accepted to be the early training of the Primarchs and their testing and forging in to weapons of war. Character building are probably the words I'm looking for. But also, something of their home worlds probably seeped in to them during this time. Indeed, the Isstvan campaign for the Horus Heresy from Forge World (i.e. Betrayal) specifically notes that something sinister seeped in to Mortarion's soul from his time on Barbarus. And it certainly goes without saying that the Khan inherited his fighting style from his home world, if not his appearance.
But beyond this grounding, I think there is further cause for the complicity. It made the Great Crusade much easier. Every primary (barring the missing ones and barring Angron) were able to unite at least one world (or in the case of Alpharius: probably a fleet) under their control. Consider Roboute Guilliman's achievement: 500 worlds all forged in to a great empire stretching the length of Ultramar. This singularly must have made the Great Crusade an order of magnitude quicker. Rogal Dorn brought the Inwit cluster of worlds under his control. And so forth. Even if Mortarion did not have multiple worlds under his control, he certainly conquered all but the strongest necromancer on Barbarus and brought unity to his planet. All of these conquests add up and must have contributed to the advancement of the Great Crusade and its fuelling - both in terms of raw resources, and humans. So perhaps it was a reasonably good idea to allow the scattering in hind sight. Just don't mention the subsequent heresy...