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, please 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.