My first time using the Note series and the Spen, was with the note 2 in 2012 that I had together with my n9.
Note phones come with active wacom digitizer and pressure sensitive Spen.
I won't talk about the BT, gyroscope and remote use functions that note 9 and 10 have on the Spen, only the general basic features. I'll just give you an example for my usage scenario using one of those features at the and of the post*
- 4096 pressure points.
If you are an artist, you really appreciate the precision that the spen gives you. There's too much to talk about here, but you get the idea.
- Notes
You can take notes without even opening the screen. Take the spen out, write in standby screen, put it back, your note is saved. Later you can just tap on the hand written text and have it instantly converted in digital text. Even if your handwriting is bad, the recognition is really good.
- Text selection and extraction.
Some pages don't let you copy text with a long press. If you take the spen out and click&drag, you can select it without any hustle. Also if you have some images, screenshots or you just select one portion of the screen, you can click extract text and have it instantly in digital format ready to copy, save, send or translate.
- Spen Suite
Write on pdf, fill documents, sign them, etc. I get many pdfs on my phone and I can edit them, sign them, send them without actually having to print-write-scan-send, cause the spen makes them look like they were actually hand written.

And many other features like hovering over text with spen to preview links, dropdown menus, translation, magnifying the screen, cropping any portion of the screen fast/precise and much more.

*And the remote function example from above.
I'm doing game pitch presentations on a weekly basis.
The presentation room has a good dell monitor on the meeting table and a shitty projector that everyone is using to connect their laptops (and then stand over them to click next/back).
I just connect my note 9 with a type C to HDMI cable that I left on the monitor and I'm instantly in dex mode (desktop environment on my note 9, using external display and every external device I want).
I open the presentation, take the spen out and i can move around the room freely cause i can control my presentation with the spen like a remote.

So yea, that's what i do mostly with my spen!
Ps. I know that most people nowadays buy Note phones just for the hype and they use the Spen to scratch their head or other places, but as you see you can do really great things with it