Being King gives the character various effects like + influence, + troop morale, + personal security, + tax collection... ect.
The effects depend on the title, for example, the king of France obviously will have more and better effects, than the duke of Bohemia.
In order to be the king of something you must control certain settlements, like the king of England who must hold on to London in order to keep his title. Like wise lets say you have the king of england, now if London where to fall to another faction then that character will get the trait NOT king of England which gives the character negative effects until London is recaptured.
There are also more titles that characters can get than what they start with.
Like,
Taking Scotland as the English, which gives your faction leader " king of Scotland ".
Taking Jerusalem as a crusading faction, gives your faction heir " king of Jerusalem ".
Taking Rome as certian factions, gives your faction leader the title " protector of the Pope ".
There are many more titles, but I'm not telling so you'll have to find them

.