“Just add one more sensor”
During installation, the customer's engineer asks a technician: “Can you just add one more sensor at the infeed? It's small.” The technician wants to help.
But one sensor means a new I/O point, PLC code, a drawing update, testing, maybe a safety review, and someone has to pay for it. If every “small” request is done informally, the project slowly drifts from its plan, its budget and its contract.
Integrated change control is how a project says yes, no or later to change in a controlled way.