Well I think the purpose is to finally have serious questions answered under oath. Call it leverage. If he slips up and tells too much the information can be used to find Natalee. Joran has given at leat 3 different stories about that night. Lying under oath here is a crime no matter where you come from. If he tells the truth it could help find her. If he lies under oath and is caught...well you know the rest.
I notice that there is not a wrongful death count....that is interesting.