I have never understood the desire or need for these. Why not tackle whatever problems are in the way of what you are trying to make? I guess some people like them for their own sake, but I haven't ever felt that way.
Some of these, at least, are excellent ways of learning how to think about a solution to a problem. Take the Friday the 13th puzzle for instance -- you can do a lot of floundering around with a puzzle like that in some languages until you realise that it's exactly equivalent to looking for all of the Sunday the 1st dates, and it's been a long time since I worked in any language that didn't have an easy way to do that. Sure, some puzzles are just puzzles for the sake of cleverness (the typical "think outside the box" things), but others are about decomposing or restating a problem to make it much simpler that a frontal assault on the original problem as stated. If you're the sort that can immediately intuit the decomposed problem all of the time, cool -- most people need to train their minds to work that way.
