It doesn't take much. A neighbor and I just had to deal with a bunch of property line confusions in the US where a property subdivision 25 years ago wasn't handled correctly. None of us really cared but it would matter in the event of an eventual sale and it cost thousands of dollars in surveyor and legal fees.

ADDED: We also had a bunch of easements and cooperative maintenance agreements that were only partially documented in the deed and mostly done via a handshake. So we got that all squared away in the expensive binder from the lawyer.

The public public land record in Croatia is notorious for being outdated and poorly maintained. Basically, the last time it was properly maintained was during Austria-Hungary a century ago, or at least that's the popular opinion. Right now we have the cadastre and the land registry and they do not always align. And for your example of unofficial agreements, the land my father and his brothers inherited is still in my grandfather's name, he died over 20 years ago, now my parents are aging, and there is just a verbal agreement on how the property is divided.

This is well documented in the the book "The Mystery of Capital" by Hernando de Soto. When it was published I thought it would change the world, but apparently nobody cared. :-)

In his thesis, this is the reason capitalism cannot work well in Latin America and other nations around the world. He says that registered land ownership is the foundation of capitalism. This is how one can borrow money against your land and invest it to make more capital. Very common for example with farmers in N. America to borrow against their farm, for machines, seeds and fertilizer.

(I am not an economist)

That’s how it works in the US too. We don’t record documents with the county recorder to make them official, we do so to provide notice to third parties who might purchase the land.