A King is a KIng. This means a King has more powers than a normal lord. If people swear fealty to you; you should have some benefits after all. To be a King you need 2 or more people to swear fealty to you.
1. Taxation: ability to tax your vassals. You will receive half of your vassals income rounded down each turn.
2. Divorce; as a king you have the ability to grant divorces for any of your vassals family.
3. Settling disputes; As a king it is your job to make sure your vassals are okay with the plan. If two people want to colonize a piece of land it is up to you to make things even. Be ware that this could lead to your downfall.
4. Peace; only a King can sue for/offer peace to an enemy force. This again could lead to contention with with vassals.
5. Trading; as a King you get perks for trading. A King can give/receive as many resources as he wants from his vassals.
6. A King can swear fealty to another King. All of the vassals who wish to remain will be transfered to the new King. As this is a more peaceful solution the King that is swearing fealty will not lose as many points.