I've searched for it a bit but so far I've seen no answer. All I see online is the discussion about fence breaking... But what's the use of fence actually? Even the Wiki doesn't seem to contain an answer. I remember reading that they would keep wood and grass out of an encircled zone. Is that correct? But can it stop the plowed land from becoming normal soil again?
Fences will restrict the spread of grass and the movement of animals, so they're primarily useful for making pastures. That said, there's not really any harm in having your animals roam the whole farm except for making it slightly hard to find them. Other than that, there's no concrete gameplay benefit, but partitioning your farm into areas can make it more aesthetically pleasing and can help you with keeping tabs on different parts of your farm.
Since fences are just impassable tiles, they have no special effect when making a closed circle; they won't stop wood, stones, and grass from appearing inside a fenced off area, and they won't stop tilled soil from turning back into normal ground. There is technically nothing special about a fence that's not shared with other unpassable tiles.
So you are correct, there is very little information online regarding fencing. Here is what I've learned from playing the game so far:
Fencing will stop grass from spreading and other pieces of harvestable resources from appearing within the fences in area.
Fencing in crops or plowed land will not stop the land from returning to "unplowed". This is a matter of the land being used or not.
Finally, I have noticed that if you don't fence in your crops you will randomly lose part of them due to "animals". I believe the game mentions this mechanic briefly in the beginning.
These are all trends I have seen with fencing my crops in and results I have suffered from not fencing my crops in as well 😐