When you hold a pair of nines and the dealer also reveals a nine, your gut will urge you to stand since you have a hard 18 and cannot win. Your chances of winning in blackjack are more significant if you split rather than stand.
With such a hard 18 up against a 9, your odds of winning are eight out of twenty. Not a terrible set of odds, but if you separate 9s, each 9 will have a chance of garnering 9.5 winning hands in blackjack out of every 20 hands. This indicates that by dividing your 9s in this instance, your chances of winning are 1.5 times greater than if you did not split.