A king title is essentially just a larger version of a Duke.
Rather than dealing with a huge conglomerate of Dukes, and Counts who are unruly, you can put them all under the banner of one guy, who keeps them in line for you. So rather than having to maintain good relations with a dozen characters, you can focus on making just one guy happy. The flipside is that he's a very powerful ruler if he rebels against you, so it's a bit of a double-edged sword. King titles do add a lot to your prestige though.
Generally I won't create them unless I have an area that is difficult to administer because of geography, or cultural/religious differences (i.e. Sicily + Southern Italy when I play the Byzantines, I'll create a Kingdom of Sicily). I'll often only create one Kingdom as an Empire, and will pass that Kingdom to my heir. That way I don't have to worry too much about him rebelling, and when he succeeds I am able to readminister the Kingdom as I like before passing it on to the new heir.