That said, we can try to detail what aspects/purpose each Primarch embodies.
#WARHAMMER 40K PRIMARCHS FREE#
The purpose of the historical articles J-files by Jervis Johnson was to encourage players to free their creativity and create the fluff for their own army. The Eldars Craftworlds, the Orks clans, the Imperial Army regiments are examples of this philosophy. We should remember that 40k is before anything a game and, from a gameplay point of view, the philosophy of Games Workshop is to create a generic, vanilla version of each race and army, and give players enough flexibility to play with their own style. More than having a specific purpose designed by the Emperor, each Primarchs represents an aspect of the perfect soldier and ruler the Emperor is. Fittingly, the powerful psyker Magnus grew up on Prospero, a world far from Terra where psykers could hide and create a society emphasizing knowledge, while Gulliman was educated on a planet where the elite were keen on discussing politics and law. Some had a definite purpose, but it is hard to tell if it was planned by the Emperor from the start, an inevitable consequence of their creation that each of them would be heavily specialized, a consequence of their dispersion by the forces of Chaos and/or their upbringing on various worlds. Besides being a charismatic leader and a deputy for the Emperor (He cannot be everywhere!) of course. Note: I am not sure that each Primarch has a defined role after the Great Crusade and the complete unification of the Galaxy as the Imperium of Mankind.