Jesus is still trying to find some peace and stillness after the beheading of John the Baptist his cousin. Life as today was full of business and a lack of peace and stillness.
Literary evidence exists showing that in first century middle eastern world people were quite interested in the ability of the gods/God to exercise control over nature and its elements. The miracle, at this level, gives divine authentication . The account also reveals Christ's mastery over the sea, not just water, but the sea seen as chaos. For the Jews the sea was a dark and forboding place, not just because of its many dangers, but because it was the dwelling place of dark powers, of Leviathan. The stilling of the sea is therefore not only Christological in orientation, but also eschatological; Jesus is even now stilling the deep. Matthew's addition of the Peter incident focuses attention on Jesus' saving power.
The gospel tradition was shaped by oral transmission such that the stories developed their own particular shape in different geographical regions and churches. When it came time to write these stores down (prompted by the increasing age and death of the apostles) the gospel writers selected, shaped and edited the tradition to enhance their own particular theological agenda.
Although there are differences between Matthew and Mark's accounts of Jesus' walking on water, the theological perspective is much the same. Both reveal divine authentication, both image Jesus' preemptive struggle and victory over the powers of darkness, and both reveal the fulfilment of Israel's messianic hope in the prophet like Moses - when even the wind and the waves obey him.
Ultimately the story is about having faith when the going gets tough. Fix on something beyond the immediate and you will ride the storm and be at peace. Wise words for today when we have to walk on choppy seas and we get frightened and start sinking!