Why are the infirm sometimes healed progressively during ministry in a meeting?

Even Jesus who had great power and great authority from his Father in heaven had to minister in two stages to the blind man in Mark 8 below before his vision was completely restored. So even for Jesus miraculous healing could at times have been a process.

They came to Bethsaida, and some people brought a blind man and begged Jesus to touch him.
 23 He took the blind man by the hand and led him outside the village. When he had spit on the man’s eyes and put his hands on him, Jesus asked, “Do you see anything?” 24 He looked up and said, “I see people; they look like trees walking around.” 25 Once more Jesus put his hands on the man’s eyes. Then his eyes were opened, his sight was restored, and he saw everything clearly. 
How much more for us his disciples who do not have the same level of power and authority as Jesus had in the gospels. We will at times need to minister to the infirm in the same way—two, three, four, five or more times until their healing is complete. But the good news is that disciples can grow in power and authority as we persevere in using it for the advance of the kingdom. “He who is faithful in a little will be faithful in much.” The Lord will “promote us in rank” and grant us greater and greater authority if we are faithful in healing the sick as we proclaim the gospel to the lost.
The important thing during a demonstration of healing is that the infirm person progressively improves following each round of ministry. Therefore it will be clear to the training participants that if we have the time and strength to continue ministering to the person, he or she will in fact eventually be completely healed. 
What is important is that our students witness the pattern of the “mountain” of infirmity progressively moving “toward the sea” in obedience to our authoritative commands.