I’m a big proponent of completely mastering the tools with which you work every day. If you work with WordPress and develop your own themes, it’s really important to understand the established template hierarchy that exists there.
WordPress uses different files based on circumstance. It depends on the URL in question as well as the files available. There’s an internal system in place that checks for the existence of certain files according to specificity and wrapping your head around those rules will help you build better themes.
The guys over at Digging into WordPress have covered this topic a number of times, if you haven’t had a chance to look over their really-easy-to-absorb guide to WordPress’s template hierarchy, I absolutely recommend that you do.