A supplement is necessary when the estimates don’t include the cost of repairing additional damage that wasn’t originally noted or seen and can even be from part price difference (PPD).
This is when an insurance company lists a price for a part on the estimate but the shop can’t find it for that price. (This is usually when an insurance company is trying to put aftermarket parts to keep costs down).
One common example of hidden damage is when the damage is found once parts are removed and finding interior components are damaged.
This is why you need to hire the best auto repair shop to handle the communication process and supplement requests, which means adjusters have to come back out to inspect the newfound damage. This can and most likely will delay the process of the repair.