Work has crept into the job that nobody instructed

Scope creeps in through the everyday courtesies of a live site: the foreman says while you're there, the drawing shows a detail nobody priced, the clerk of works prefers it done the better way. None of it goes through the change procedure, all of it costs money, and by application time the account carries work that cannot be traced to any instruction.

That trace is what pays. Most contracts make an instruction - in the contract's form - the trigger for valuing a change, and work without one starts the argument a step behind: first prove it was required at all, then argue what it is worth. The register discipline closes the gap while it is closable: every candidate change spotted, chased to an instruction or knowingly parked, so the account and the paperwork grow at the same rate.