I've been reading the Horus Heresy series and I've recently completed both The Thousand Sons and The Master of Mankind. The implication throughout the series (and again in this question) is that each of the Primarchs were created for a reason.
In The Master of Mankind The Emperor explains to Ra that his ultimate goal was to reduce mankind's dependency on the warp by using the webway. With no need of psykers, astropaths, or navigators humanity would be immune to the chaos gods.
It's hinted (again suggested in the linked question) that he planned for the 15th Primarch (Magnus the Red) to sit on the Golden Throne and operate the webway. Obviously that went slightly awry when:
Magnus, after hearing of Horus' betrayal breached the psychic fortifications around the webway in an attempt to contact his "father" and let chaos in.
However this confuses me. If the Emperor ultimately intends for himself and Magnus to be the only remaining psychers then why does he sanction him at Nikea? At this point in the series Magnus had indeed done nothing wrong!
Why was The Emperor was forbidding Magnus to do the very role he was created to do?
If Magnus was intended to be the psychic primarch then why order him and his legion not to use their abilities?