Incorrect nesting breaks the one thing markup exists to encode: which entity owns which property. Nesting is how structured data expresses relationships, so when a property sits under the wrong parent, Google can no longer tell what belongs to what, and the meaning the markup was supposed to carry quietly collapses. The complacent read, that nesting does not matter as long as the block validates, misses this entirely, because validation and meaning are not the same thing.
What makes this failure dangerous is that broken nesting often passes a loose check while conveying nothing useful. The structure can be technically valid and operationally useless at the same time. A price placed outside its Offer, or a number that is actually a SKU sitting where Google looks for a price, gets attached to the wrong entity or ignored, and the page loses eligibility for the feature the markup was meant to unlock. Competing top-level entities are worse: when two blocks both claim to be the main entity, Google may resolve the ambiguity by ignoring all of them rather than guessing wrong.
The mechanism underneath is that nesting, with its parent and child relationships and its use of identifiers to link nodes, turns isolated fragments into a connected graph. That graph is what lets Google read an author as belonging to an article, or an offer as belonging to a product. Sever a connection and the graph degrades; the data may still be present, but the relationships that gave it meaning are gone, which is why the page can validate and still earn nothing.
So the editor checks that each property sits under its correct parent entity, confirming the relationships are intact rather than assuming that a clean validator result means the structure actually says what it should.