You can use plain text for everything. Writing technical documentation and prose, keeping notes and making plans.
Word Processors. Microsoft Word, Pages.app, OpenOffice, and similar apps are suitable for creating printable documents and printing them on paper, but not for editing text. These excellent pieces of software distract you from writing words. You rarely use any of their features, but they slow you down.
Cloud Services. Google Docs, Evernote, Notes.app, etc. Cloud services are beautiful and convenient, but the sad truth is that there is no cloud—it's just someone else's computer. Do you trust those corporations? You should not. They can accidentally block your account or lose your data, and you will rarely get prompt support—if you get any support—in these situations. And what if they shut the service down? Can you transfer your data out? Is the data format portable—can you open your files in another app? Can you use the cloud when you are offline? If yes, does it reliably sync your devices? Can you make backups and validate them? How easy is it to restore?
The plain text file format is straightforward and portable. You can open and edit your plain text files on any computer or phone. All operating systems support plain text natively and will always support it. You can edit your files with any text editor—no need to convert, import or export anything. It just works. Everywhere.
Plain text files are small and easy to manage. Plain text tools are powerful. With plain text you get superpowers of version control, rendering to printable documents and web pages, and simple backup, restore, and encryption.
On macOS? Try TextEdit and Vim. On OpenBSD? Try vi.