If you have ever written a standards draft, they are exclusively in plain text.

For examples, load up http://www.faqs.org/rfcs/rfc821.html and start scrolling.

I do a lot of geometry related programming and the ability to add some graphic illustrations directly into the source code comes in handy every now and then.

Also, everybody needs a small ascii graphic in the output from time to time.

EDIT: Language.

It's a clever tool for creating Ascii art. Not for everyone, but if you've ever tried to create boxes and lines in ascii, this is a tool that makes that so much easier .

This is something I might use in source code to describe / draw the relationship between elements.