Why is a diff not enough?#
A diff is complete and meaningless. It states every line that moved and nothing about what is now true that was not true before. The reader has to reconstruct the meaning themselves, which is exactly the work the delta exists to do — and exactly the work that does not happen when reviews arrive faster than they can be read.
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Why say what did not change?#
Because a reviewer who does not know what stayed the same has to check everything again. Stating the invariants — the properties that held through the change — is what makes a review proportionate to the change rather than to the codebase.
A delta is written against a recorded starting point: what this person last knew. That is what the re-entry brief builds on.