I'm probably underselling it but for me, markdown support and note organisation simply reflecting the directory structure is what sold me. I can then sync it whichever way I want; I've taken to having a "notes" folder in each project and including it in the Git repository. The notes are then browsable on GitHub too.

I've spent a lot of time trying to find my "perfect" note taking app, and have attempted to build my own a few times, and I'm very happy with Obsidian.

They're the same in the sense that Mail.app and Outlook are the same.

Apple Notes is great, but can't do things like tagging/graph view and doesn't allow you to export in a standardized format.

Obsidian is a superset of Markdown (with a few modifications for page embedding and wiki-links) and allows you to sync with anything that provides a way for you to upload a folder of text files.

It’s a much more fully featured note taking app, probably most directly comparable to Roam Research.

E.g., tagging, graph views, back links between notes, note/document templates.

All docs are (more or less) plain markdown as well, so if for some reason the open source community ever abandoned Obsidian, in theory it’s easy to export/transfer your notes.

Well it is open source, is cross platform and is not made by Apple. So, really different.

It is actually not currently OSS, although the creators have indicated interest in open-sourcing parts of it in the past.