This is will allow members of my team (or myself) to get to know the candidate, evaluate the 'wave lengths', and how effective he/she is at finding patterns on the internet/books -- rather than thinking things up.
It also demonstrates to the candidate commitment on our side, and it naturally forces us to find problems of a proper size/effort (as we are spending the effort too).
We would not do it for all applicants, though -- only for once that pass basic screening / competency process (there are no trick questions or exercises there