The Old Testament gave us many kingdom images. The King created the world with just a word. He commanded the flood to come, commanded the Israelites to leave, commanded the sea to open, sent His children into exile, and pulled them out of exile. God is creating a kingdom.
In the New Testament, Jesus quiets storms and controls demons to give us an inkling of what life will be like when the King is on this earth, where He will once and for all silence all storms and banish all demons. The purpose of the miracles is to give us incontrovertible evidence that Jesus is something other than anyone we have ever seen, worthy to be called king. He proves his kingship now so that we, longing for a king, will turn to him as ours.