A proposal is a record of answers. It isn't a record of how you got to them.
Delivery inherits the document and starts work. A few weeks in, someone asks why it's built this way, and the answer isn't written down anywhere.
What usually gets lost:
- which constraints were firm, and which were a preference someone mentioned once
- what the customer means by a word like "available", which can take two meetings to pin down
That isn't an accident. A proposal is written for the buyer, and the buyer doesn't need any of it.
So hand over with a conversation as well as the document. Otherwise delivery rebuilds your reasoning from scratch, usually after a decision has already gone the wrong way.