Where possible, I try to write everything in plain text files. It has many advantages over word processing software like Word or Google Docs. The files are lightweight and tiny. But it's also future proof. There's no additional steps needed if you are working between systems or if you open a very old file.
Perhaps the biggest reason I write everything in plain text is it is boring. There's no additional styling you can add. A lack of formatting is actually a big selling point, because it is a distraction free file type. Instead of choosing the perfect font and aligning things just right, I focus on writing the words.
Emacs is a very old program that saves everything in plain text. I use it for Org mode, which is about as fancy as you can get for text on a screen. But any software will open .txt files, even notepad which is installed on windows by default.
- This is a useful piece of software that will combine multiple text files.