Text documents are pure, unadulterated text. No fancy stuff at all. I use Textpad quite a bit, though freebies like Notepad are excellent.
When I started writing web pages I used Textpad exclusively. Such programs don't through in any extras (like bold or italics) that are certain to interfere with the smooth running of a web page
Even now, when I no longer write web pages, I keep a link on my taskbar. It's very useful if I want to by-pass the site formatting if I want to add something to a Blurtit answer (for instance, if I'm quoting a limerick and don't want double-spaced lines).
I can't be specific about your particular question, Matthias. I did use it back before Windows when we were all working with DOS and had to make up things like batch files to operate the computer. These days, you'll occasionally need to use a text file to remove malware from your computer. When you find yourself in that situation you'll get instructions from an anti-malware site about how to use it.
