One definition of the word "testimony" is: God will do it again. Hearing testimonies from people who received an answer to prayer, or a need met, or a physical healing in their body encourages us to trust God. It helps us bridge the gap between the impossible and the unsure, to the possible and God will do it for me too! Faith comes by hearing, and hearing by the word of God (and testimonies of people who stood on His Word and received what the Word said they could have).