Why was a dragons head carved at the front of the longboat?

Because vikings thought that it would strike fear into the hearts of their enemies as they sailed in for war. Viking myth described a cosmology where a great malevolent worm, Nithhoggr, lived at the base of the tree of life trying to break out of the dark and feeding on the souls of the damned.

Later legends such as the Tale of the Volsungs also incorporated dragons (Fafnir) as fearful creatures of power to be overcome by heroes.

As with many myths and magics real pagan Vikings often would invoke the strengths of these mythic creatures by using their images on longboats and jewelry and weapons.