Accurate serial number tracking is the reason for locking them in the system. It’s not accurate if you can’t put a returned and repurchase serial in the system without adding a modifier.

Example: Aug 12 you bought a water heater for a customer, Customer then decided to do something different. You return the water heater on Aug 25 and posted the return. This should have cleared the serial for purchase again.
Nov you require the same product and your supplier sells you the same Serial again. You can’t post the Nov purchase without modifying the serial #.
Not a good situation if in three years you need to make a warranty claim, or if someone has fudged something along the way.