For notes, I don't see the appeal of having a browser interface. I just put my notes in a text file. No protocol, just search for text strings or text tags. If I need hierarchical organization, I use directories. What am I missing out on?
1. Non-technical colleagues.
2. Easy and real-time collaboration.
3. Slickness, mixing of multi-media, etc. This is related to 1.
Not arguing against your preference, just answering the "why".
1. Notions "Databases" are easier to use, edit, and manage than any similar setup I could figure out locally. And they have inbuilt integrations so you can sync things like Github/Gitlab/Jira, etc directly into your docs. I highly recommend setting up a personal project inside Notion to try it out.
2. Less important but still useful, the collaboration is good to use with my partner. We can have shared household notes/saved info/tasks/etc all in one place very easily using it.
It lets you structure your notes in a way that's reminiscent of object oriented programming (you've got templates to define types, inheritance of attributes to instantiate and specialize them, etc). I have hierarchies of hundreds/thousands of notes that for all intent and purposes are as good as Notion's Databases
Google docs is also browser based? And obviously not foss or self-hostable.
Are you saying "why does Google have docs app in addition to a note taking app?" (Google docs vs Google Keep, Microsoft Word vs Microsoft OneNote etc?)
I don't see the appeal of having a browser interface.
Browser level access allows for multi-device multi-context (personal/work) access.
Also google docs/drive is deeply unusable when trying to organize mental spaces.
Great until you need an image or more than the simplest table, as examples.
How do you embed an image? ASCII art?