https://itch.io/games/prototypes
https://itch.io/games/in-development
Lot's of games are published without being finished. I can "follow" your game by putting it on a collection. But practically I would follow the developer in any case.
I do not understand the question. You can sort the in development games for top-sellers, and you can look at the top-sellers regardless of publishing status. You will find non-finished games there.
You have to consider your audience. And who the developers are. Releasing early is most of the marketing a small or amateur dev does, because it costs nothing. Also, you might get a sounding board how your game will be received and get free alpha testers.
It also depends on the fact, if you have a publisher or not. Itch is self publishing and low no entry bar. So one could say, people do it, because they can do and the real question is, why not everyone does it so.