Equiano's witness testimony helps Wilberforce in two ways.  First, Equiano shares his experience with Wilberforce of being captured as a young, wealthy African boy, sold to slave traders, enduring the inhumane conditions of the slave ships bound for the islands and America, and his struggle for freedom afterwards. To be able to hear a firsthand account from someone such as Equiano has a tremendous effect on Wilberforce and provides further encouragement to pursue his cause to end slavery.

Secondly, Wilberforce goes down to the docks to see what the ships look like, how cramped they are, and what conditions the men and women on board the ship endure.  Because he sees with his own eyes--at Equiano's suggestion--he can no longer deny even in his mind that the toll is unnecessary and unconscionable.