"Stay on top" / "Always on top" i.e. keep the window on top of all other windows. There was a tool called Afloat which worked for previous macOS versions but does not for macOS Catalina.
Example - If I am reading a PDF or an article on a webpage, I do not want to constantly find my note from behind the webpage / PDF to type.
The native Notes app has this functionality (see screenshot) too. Also, have added another screenshot from Bear which shows a little symbol on the bottom right of the note which toggles the "Always on top" or not - which is pretty handy.
So, all in all, looks like the OS does have the API / functionality to do this (given Bear + other apps use it). I would love for Evernote to implement this please!