The man himself has the seed of faith nurtured by his encounter with the Lord. He offers that small seed to Jesus for Him to water and grow it into a much bigger and stronger faith. His son is given freedom from the demon. Everyone who witnesses the miracle is amazed by God's glory (Lk 9:43). And the disciples learned a valuable lesson on dealing with evil spirits, which I am sure they needed to know later in life in their work with the early Christian Church.
How strong is your practical faith? We can believe in Jesus as the Son of God and Creator of all things, yet not believe in His power or His willingness to suspend the natural order of creation for us in a particular situation. Personally I am most guilty of believing in His ability but not in His loving willingness. The man described in this account wasn't sure of Jesus' ability but he was willing to believe and willing to be transformed. Think of one situation in your life right now in which you do not have practical faith for the Lord to work a miracle. Ask the Lord to transform your unbelief into faith with an open heart to see Him work in amazing ways. Ask Him to build up your faith, then expect to see Him answer that prayer.