So you can choose to not use and save perk points when you level up. I'm not reallly following the point of saving them though to be honest. If you have two points to use the next time, isn't it exactly the same as using it the first time? I suppose the only point i can see is that if your skill is currently too low and you want to save the perk for when you meet the requirement to use it ? Yes?
You answer your own question.
That is exactly why.
And you don't need to wait to level up again to use the 2 perks, you can apply a perk whenever you'd like.
If you HAD to apply a perk when you leveled up, it would be an awfully bad leveling system.