Almost completely pure way of storing ideas. With this I mean that you don't store unnecessary data such as "background should be white" or "left page margin is 1.3cm". It's just text. What's important is what it says + minimal markup.
Presentation is left to the reader's client. Do you want dark mode? Get a markdown editor/reader that supports it. Do you want serif font? Again, that's client's choice and not part of the document.
Markdown is good. I use it when working in the company since the format is ubiquitous. I do writing my blog posts with Markdown (Hugo for the curious).
But personally, or working with a bit more niche team, for writing personal documentation I prefer Asciidoc [0]. It has better syntax and have some nice functionalities like Table of Contents.
For personal notes, nothing can surpass Org Mode [1].
I was looking for a journaling app that didn't have vendor locking, or required some weird export dump that messed your formatting and folders up. That lead me to Markdown and Obsidian. I love it. And when I die, that shit will still be readable by any basic text editor.
Eh, while Markdown is nice I think Dokuwiki's syntax is infinitively better for any kind of text that ends up involving programming code. It also has a header syntax that makes sense, albeit rather cumbersome. And it also makes a proper distinction between italics and underline which are two different, standard typographical effects and not the same thing as Markdown seems to believe; and between ordered and unordered lists (let alone nested lists).
Just about the only bad thing is I haven't been able to find an editor that supports it. Probably because, to my knowledge, no self-standing / independent renderer exists for it (the parser and renderer seem to be tightly integrated into the content manager).
One of these days, someone is going to invent a confluence alternative that only supports markdown and doesn't have nay of confluences stupidity and it is going to EXPLODE and bankrupt Atlassian.