Because it's trendy, and Facebook did it! But, on a serious note, it shows a little bit of drive and willingness to put forth some effort into your application

Because it's trendy, and Facebook did it! But, on a serious note, it shows a little bit of drive and willingness to put forth some effort into your application. It also serves as an initial filter, helping the company to target potential employees that perhaps think like they do or, something. It also filters out people that can't actually program simple solutions. How is it not useful to have some sort of programming challenge?