The purpose of "But God, You Promised" is to connect with the heart of believers who are experiencing a "silent" God in the midst of a wilderness wandering, waiting for their promise to materialize. It is a peek into His process of development: when it appears nothing is happening, is when everything is happening.
By examining the life of Joseph, we will see that it is this very promise from the Lord that becomes the instrument of our purification and development. It is in this long wilderness time that our character and skills are developed, preparing us for the fulfillment of His promise.
But God, You Promised is to nudge your faith, encouraging you to keep on keepin' on! God has you right where he wants you!