Guilliman had grown quite intrigued by the Raven Lord's abilities as a shaman. Taking souls and binding them with suits of armor was an amazing application of soul manipulation.
Not only did it allow one to control soul apparitions independently, it lessened the toll on the shaman's soul by granting the soul apparition another source of energy.
Of course, what the Raven Lord could do was much more complicated than that, since the souls seemed to have the ability to respawn even though they were connected to an external source.
It was a lot to digest, so he decided to take it one step at a time.
First, as the holder of the heavy soul, he didn't really need to manage his soul energy expenditure like the Raven Lord, since he had a literal bottomless well.
However, he could use the other method, which was to bind his soul beasts to an object—or weapon.