Interoperability. I'm of the belief that the vast majority of files should be stored in plaintext or human-readable formats to prevent lock-in and make it easier to parse their data, and while a lot of the apps I use on a regular basis respect this idea, there are a lot of apps that have completely thrown it out the window. If your program manipulates files and has it's own file format, <i>please</i> consider how it looks when you just open it up in a text editor point-blank. Unless you're dealing with password encryption or secret obfuscation, there's really no reason not to go the extra mile and make your files not appear as gobbledygook.