When it comes to Markdown its main advantage is not its features (it does not have many), but its simplicity (and ability to edit it in a text editor - blazing fast, no distractions, etc). And then I can convert it to anything.

Vs LaTeX - sometimes I need just to note (a to do list, a draft of a blog post, an outline of workshop), without the boilerplate or visual noise of LaTeX.

(And I wrote more than a dozen of papers in LaTeX - so mine argument isn't "LaTeX is too hard".)

That said, I enjoyed using Overleaf - but mainly for teaching LaTeX (no installation, very nice and readable error messages, nice examples (I use your default page as the first introduction)). But for anything longer it's a pain that everything needs to recompile every few keystrokes (especially on a slow connection); I know it's not you but LaTeX; but for the same reason sometimes (when not requiring full features of LaTeX) it's easier to use Markdown + MathJaX.