1. You gain prestige.Now I see no reason to usurp a kingdom.
Well personally, number 3 is the one of the most useful ways to expand your kingdom when you have got to that level of power. There's also the fact that if you hold ridiculous numbers of duke titles then all of your vassals will hate you, which will screw up your tax income and make it hard to change laws.Now I see only one reason because:
1) You gain a lot more prestige holding a lot of duchies without being king
2) If you can usurp the title you are probably stronger than the king already
The third option is situational and not always justify to usurp it I think.