There are two reasons those parts are listed in an estimating guide. One is to insure they don't get overlooked if there is damage that isn't readily obvious. It's easier to tell a customer some of the parts on the estimate weren't needed and the final bill is less than expected than to have to tell them more parts were needed than originally thought. The other is if any of those parts contain a crash or impact sensor. Those sensors may appear fine on the outside, but the concern is there could be a tiny arced spot where a spark occurred when contact was made, and in the event of another crash, that arced spot could prevent a good and solid contact from being made again and that bag won't deploy when it's needed. It's a liability thing. Electrically those sensors will still test okay during the system's self-test at engine start-up, but there's no way for it to know about the integrity of the contacts. Body shops will replace all sensors that tripped to avoid any liability or future lawsuits. Many do-it-yourselfers leave the old sensors in because the system will appear to be working correctly and they don't understand the reason they're being told to replace them.

They may also want anything replaced that has electrical connectors going to air bag assemblies. They can be burned from the burning rocket fuel and may not seat properly in the new assembly. Also, if there's anything less than a perfect connection, that circuit could test okay with the very small current during the system self-test, but it won't pass the high current when it is supposed to deploy.