Client-supplied materials
You keep control of choice and price, and you carry the risk of a shortage, a defect or a delay. That is a reasonable trade as long as it is agreed rather than assumed.
Contractor-supplied materials
We carry the risk and the sourcing. The estimate states which items these are, so nobody discovers the answer when a box is opened.
Why a change is written down
A verbal change is a disagreement waiting for an invoice. Every change gets a written scope and a price before it happens.