I agree that it's stupid, but it's an easy way to make money if you're popular enough to get views. The phones are video props. They cost a lot less than the the video made. It has over seven million views, which is what determines the amount of money it makes. The goal for this type of video is to increase subscribers and get YouTube partner status, if they don't already have it. They can make quite a bit from ad revenue. YouTube partners can easily make $250,000+ per year.
It seems stupid when you look at it from our point of view because we just see it as destroying a new, expensive phone. But if you look at it from the video makers' point of view, you'll see that it's a way to turn a small amount of money into a larger amount. Destroying perfectly good iPhones tends to really gets people's attention.
tl;dr: It's sort of like blowing up new cars in action movies, just smaller scale.