My original description of what it is still stands: “Markdown is a text-to-HTML conversion tool for web writers.

Yes, that's how John Gruber defines it. But like every creation you share with others, it can evolve. If popular, it will evolve. People create their own versions and uses and intentions with it.

I've used markdown for 12 years or so, for two reasons:

1. As a way to write plain text but still get visual layout cues without using a proprietary format/tool (e.g., Word).

2. Have options for later conversion to other formats/outputs (for Gruber, HTML.)

So for me, writing markdown on my Apple device means that instead of using Apple's proprietary format, I have another place to write plain text markdown and use/share it elsewhere (which I often do).