Miracles have never appeared with any regularity. They have showed up only at certain times—only when God saw that they were needed. Hence, they were confined to certain periods of history and were wrought by only the most faithful of God’s servants.
Of those miracles recorded in the Bible, including all the miracle healings, there were about 120 in all. If you consider the great span of history, that is really not many miracles (although, there have been many miracles that were not recorded). Hence God placed them only where they were needed. And as history would indicate, they were grouped mainly in the time of Moses and Aaron, in the time of the prophets (especially with Elijah and Elisha), and in the time of Christ and the Apostles. Between these periods there were long stretches of history (hundreds of years) that were bare of any supernatural events.
Therefore we can see through history that miracles are not God’s usual way to communicate with us. They were used only at special times for special purposes. Beginning with the miracles wrought by Moses and Aaron, the purpose of these was three fold: (1) to help deliver Israel from bondage in Egypt and to sustain them in the wilderness; (2) to help give them victory over all the forces of evil in the Promised Land; and (3) to help establish the worship of the true God and the foundation of Christianity in the midst of an idolatrous world.
Centuries later, during the time of the prophets, especially by Elijah and Elisha, miracles were worked. During the years of Elijah and Elisha the purpose of those miracles was to keep the true worship of God alive, since at that time Satan was trying to destroy it with rampant idolatry. Likewise, around the time of the captivity miracles were needed to help revive the faith of the chosen nation, which was on the verge of distinction.
Again, from the time of the captivity to the time of Christ miracles seemed to vanish. But when Christ came the greatest period of miracles began. But even Jesus (the one who was God) did not do miracles until the last three years of His life. They were reserved for that special period of history, during the time of His ministry, when He set out to tell the world who He was and to declare His mission. Hence, the purpose of miracles by Jesus was to show those around Him that He was God and to help establish His authority and power as from God.
During the ministry of Jesus, sometime after He appointed His twelve disciples, He called them together and gave them all miraculous powers. Hence, during Jesus ministry, but I think even more after His resurrection, the apostles did great miracles just as He did. The purpose of their miracles was two fold: (1) to be a sign of their authority as a true apostle (Acts 14:3; 2 Cor. 12:12), and (2) to help them establish Christianity, which was the work committed to them by the ascended Lord (Mk. 16:15-18; Rom. 15:18-19; 2 Cor. 12:12).
So I think from this you can see the great and important purpose of miracles. They were done to help the people of Israel, God’s chosen people, when they were on the verge of extinction. Overall, they were done to get the attention of people: to bring them under the authority of God’s appointed leaders and to point them to the truth.