Why did the Green Knight come to Camelot?

The Green knight crashes King Arthur's New Years feast. All the Knights are chowing down when he comes in uninvited. He is basically there to challenge King Arthur. He calls the Knights inexperienced children in need of testing. Their testing ends up being not so much their battle skills but rather their code of chivalry. A knight can never let his King fight so Sir Gawain accepts the challenge in the King's place.