Most of the time anyone with solid engineering background, industry experience, programming contest medals, or entrepreneurial streak would have no problem landing an interview.

However, that leaves a whole lot of candidates out of the picture. Faced with limited recruiting resources, puzzles are about the next best thing to serve as a recruiting filter. Plus some people send them in without being told to, which is usually a plus, as it implicates coding in their spare time is something they enjoy doing.