>Can anyone tell me an actual reason to use Org mode over Markdown?

I think the reason in general is simply that org-mode is better, i.e. more carefully defined. I'll mention the things I like more specifically:

- Link syntax in markdown is something I never remember how it goes. Org-mode is [[link][description]].

- Text styles in org-mode make more sense. In markdown, * is used for italics, * for bold. Additionally, * is for lists, which might be confusing.

- Markdown blocks are freaking awful and seem to work by luck more than anything else. Code blocks even worse -- they are made by indenting 4 spaces or a tab! I couldn't come up with a worse way to do that even if I tried. Org-mode has clear #BEGIN_xxx / #END_xxx blocks.

- Markdown has a weird YAML frontmatter syntax. Org-mode has keywords which can be attached to the whole document and properties, which can be attached to every heading.

- Org-mode has tags.

- Org-mode has timestamps.

> Link syntax in markdown is something I never remember how it goes

I can remember it fairly easily. [Link](url). Very unconvinced by this.

> Text styles in org-mode make more sense. In markdown, * is used for italics, * for bold. Additionally, * is for lists, which might be confusing.

* for lists is what I would naturally use so I think that's better than - (but Markdown actually supports both). I'll give you * for italics though. Definitely should have been bold.

> - Markdown blocks are freaking awful and seem to work by luck more than anything else. Code blocks even worse -- they are made by indenting 4 spaces or a tab! I couldn't come up with a worse way to do that even if I tried. Org-mode has clear #BEGIN_xxx / #END_xxx blocks.

Sorry but there's no way #BEGIN_xxx is better than ``` or 4-space indent.

> Markdown has a weird YAML frontmatter syntax.

It doesn't. That's some extension. Maybe Pandoc?

> Org-mode has tags.

Yeah they look kind of useful.

> Org-mode has timestamps.

Can't say I've ever remotely wanted that feature...

Overall it seems like a wash.