At that level there is no such thing as 'qualified candidates' like in a generic job where you just fill your positions.
There is no justification needed (in the traditional sense), but there was one anyway: board set a goal, Tim had to meet that goal, and then they'd give him the money.
Say Apple made a couple hundred billions under Tim Cook's leadership and the way was paved for more of that in the coming years (keep in mind that there is no day-to-day, this really is many-year-planning; you practically have to predict the future to some extent), is a fraction of that really such an odd case to pay the person that did that?
Sure, you don't need that much money to live comfortably forever, but this is capitalism, I doubt the value created was needed either. But the board and Tim agreed and met their goal and thus the transaction was completed.