For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
          Gideon Mushininga
        This is an inspired question worthy of good discussion, especially nowadays when 'prophets' are mushrooming everywhere. 
Exodus 4:8 A sign has a voice behind it. An advert is not a end in itself but the real thing behind it. 
Mark 16:16-17 Miracles are given to believers. Believers who should preach the Word to the sinners. 
In conclusion, a miracle is a sign to the Word of God.
          Taniela Haunga
        Acts 2:22 KJV
Ye men of Israel, hear these words; Jesus of Nazareth, a man approved of God among you by miracles and wonders and signs, which God did by him in the midst of you, as ye yourselves also know:
Mark 16:19-20 KJV
So then after the Lord had spoken unto them, he was received up into heaven, and sat on the right hand of God. [20] And they went forth, and preached every where, the Lord working with them , and confirming the word with signs following. Amen.
          pj edelman
        In the Old Testament, with few exceptions (i.e. The Burning bush) miracles were for the purpose of helping individuals  or the nation of Israel accomplish tasks or reach goals set by God.
In the New Testament, miracles verify or attest to the authenticity and authority of the apostles and God's sovereignty over the church