One clear purpose of miracles was to authenticate the character of Jesus and his relationship with his heavenly Father. In this regard, miracles demonstrate the following:
God is with Jesus (John 3:2); Jesus is from God (John 3:2; 9:342-33); God has sent Jesus (John 5:36); Jesus has authority on earth to forgive sins (Mark 2:10-11; Matt. 9:6-7; Luke 5:24-25); Jesus is approved by God (Acts 2:22); the Father is in Jesus and Jesus is in the Father (John 10:37-38; 14:11); in Jesus the kingdom of God has come (Matt. 12:28; Luke 11:20); and Jesus is the Messiah (Matt. 11:1-6; Luke 7:18-23) and the Son of God (Matt. 14:25-33).
A second purpose of miracles was to authenticate the message about Jesus. This was the major function of the miracles as far as the ministry of the apostles was concerned. Mark says that the Lord "confirmed his word [that the apostles preached] by the signs that accompanied it? (Mark 16:20). When Luke was describing the ministry of Paul and Barnabas at Iconium, he said that the Lord "confirmed the message of his grace by enabling them to do miraculous signs and wonders? (Acts 14:3).
Notice that in both of these texts the Lord does not confirm the apostles themselves but rather "his word? or 'the message? that the apostles were preaching. Signs and wonders do not testify to the apostles but to the message of salvation preached by the apostles. So the two principal things that are authenticated by miracles are the Lord Jesus and the message about the Lord Jesus.