You are a son of the Emperor of mankind. You have a mind, and body, like no other. Superhuman in every meaning of the word.
You have an unimaginable level of charisma and ability to lead other. Combat comes naturally to you with a level of skill utterly impossible to even the best space marines and other superhumans.
All primarchs are somewhat psychic, or have the potential. And are at least somewhat protected from the warp by sheer natural willpower if nothing else.
You can also choose one power or boon which any canon primarch held, such as Roboute Guilliman's innate administrative abilities.