Simplicity, freedom to really migrate your data.

Sure, you theoretically can migrate your notes from, say OneNote to Evernote (and vice-versa), but doing so is a great experience in frankensteining. I'm not even very confident there isn't outright data loss in some places, and with a high volume of notes, it's not feasible to check manually. Even with no loss, lots of stuff look ugly and/or do not work anymore, even if there is an equivalent on the target platform.

writing your notes in markdown is a bit ass-backwards though - it's great as a storage medium but having to manually reformat a markdown document because you're not live-editing the output is a needless pain. I'd much rather write using your usual rich text interface and have that export markdown for me.

There are some editors that live preview your markdown. It’s pretty nice.

Take a look at Bear on Mac & iOS, and DayOne (my favorite but lacking full markdown support). Also Dropbox paper is interesting and live previews markdown and has nice table support

>I don't get the underlying fascination with using markdown

As a programmer, most of the notes, I take, have code snippets. With markdown notes, I get nice code highlighting in my notes.

Also a lot of times HTML/RTF notes end up having hidden invisible characters. So it messes up commands. That's why I prefer plain text notes over rich formatted notes.

I used to disagree but not anymore. I think maybe the assumption is that you will be dissatisfied enough w the app to want to use another editor, and also that's easier to write Markdown than use the UI. Assuming good exporting facilities or lack of a proprietary format and quality UX, I'd much rather have the best possible editor than be able to write Markdown. It's much more important to e.g. add images, resize them, add sketches, graphs, and/or tables, or perhaps link multiple notes together. All of this is painful or impossible with Markdown (and editing Markdown on mobile is sucky). Markdown I think is an attempt to merge markup and content in a way that retains both, but it sucks for anything beyond simple header/paragraphs/lists.

I think for most people it is portability. I can use my .md files in any Markdown editor and reasonably expect it to look the same.

If there isn't an editor to hand it is easy enough to read as plain text.

I've used Voddoopad for years. It was amazing. As a person who organizes content and research based on links instead of folders, its wiki like structure was perfect.

I had high expectations when Gus Mueller sold the app to plausible, but they haven't invested enough on it since 2013.

https://www.plausible.coop/voodoopad