Well, if you think about it, Mark is the Gospel which says “Kingdon of God”, while Matthew says “Kingdom of Heaven.” Often, a king will have a son to be the next ruler of his kingdom, so if Jesus is God’s Son (1:11) and God is the magnate (kind of like a king only He’s God), then the rendering which includes ‘Son of God’ is correct. This title is echoed again through the lips of the Centurion who carried out his death (15:39). Even legions of demons acknowledged this (5:7). And like most princes, Jesus assumed his throne (16:19) (not to imply that God the father died, but that He simply stepped aside and let his Son take charge). (c.f. Hebrews 1:3-6, 8-9; Matthew 28:18).

For a more shorter reply, most of the English Bible versions include ‘the Son of God’. If you try to take this title away (though it may be found in other passages) it’s like you’re de-emphasizing his deity.

It’s important to let the reader (like a new convert, or a straight-up infidel) know that Jesus is the Son of God. In other words, “God begat a son – Jesus Christ.” This sends a message to the lost peoples of the world that if their god claims that it never had a son (like Allah), you know that ‘god’ is not the same as Yahweh (God, the LORD). And since this statement is important to people worshiping false gods, Mark had to include ‘The Son of God’ because although Mark wrote to the Romans who were already Christians, they were surounded by many idol worshipers and false ‘gods’, and this was a way to distinguish the one true God.

Mark 1:1