It's one of the 5 pillars of Islam. One has to give 2.5% of their wealth to poor. It's compulsory unlike Sadaqah - Charity ( https://en.wikipedia.org/wiki/Sadaqah ).
I guess if everyone followed just this 2.5% rule, there wouldn't be any poor left in the world (I didn't do the math though).
P.S. When I say poor, I'm not referring to people who cannot afford the new iPhone.