And what moves God to do that?

For the alleviation of our weaknesses. If we were spiritual by nature, like the angels, we could behold God and His graces. But as we are bound up with our bodies, it is needful for us that God should make use of figures to represent to us spiritual and heavenly things, for otherwise we could not comprehend them. At the same time, it is expedient for us to have all our senses exercised in His Holy promises, in order to confirm us in them.