Have you ever tried to add something to a module, and cannot figure out why the data won't display? You could have a typo, or could be using the wrong function to access the data, token or variable you are trying to invoke. Devel gives you debugging tools that you can add inline to your code. Best practice suggests only using during development, using on production sites may impact site performance. These are much more helpful than just using basic php to output variables directly, and Devel even knows to output these things to the official message area in your theme.