The noble knight had three primary responsibilities.
The first was to be a soldier of his liege. He would ride into battle upon demand and without question. The king could always depend upon his knights being the best warriors, to their ability. This was accomplished through constant training. Not only personal training, but training of heirs - those of both the king and nobility.
Also, since the noble knight had goods and property bestowed upon him, he was expected to be a good steward of his endowments. He would go so far as to use his personal wealth to care for the poor and less fortunate. Wars and battles left orphans and widows. He considered it a duty, and privilege, to provide for both.
Finally, all knights were bound by a code of chivalry. A moral and social set of customs, that ensured an essence of purity and goodness. He would use his skills to manifest good. His strengths would protect the weak. Honor and justice were assured when he was called upon to intervene in any situation.