I remember on uni when we started defining our own formats, to later on learn about what goes inside a DBMS, and thinking, "Why on earth would I ever do the first thing again?" (not that there's no space for it, but it's not so big).
Also, when I've found a couple of times that a file that I thought had a proprietary binary format was a SQLite file, it was bliss. So much easier to navigate, inspect and work with that a huge XML file or a plain text one.