1) I started selling assets from my own projects/games I knew I wasn't going to finish. In my free time, I enjoy drawing and pixel art, and over time I decided to focus on making different card decks and, if they turned out well, selling them.
Right now, I'm only selling assets on itch.io. I've considered selling on the Unity Asset Store, but it requires a bit of effort on my part to create ready-to-use elements (or at least that's my understanding of what's expected).
For now, I just view it as an outlet for one of my hobbies.
2) In my experience, it's not really worth it. I've made about $200 before taxes in a year, though it's true I haven't put much effort into giving it visibility. I have a full-time job and this is just a side thing... I know there are people who make a lot of money. In my case, I'm not sure if the issue is quality, theme, lack of visibility, or a bit of everything 😅
I hope these answers help!