Priority code can make a difference at the dealership level. If the dealer has allocation, the priority code and order date tells the system which order to pull first. This assumes that there are no commodity constraints that are preventing the order from being picked-up. Commodities can be constrained one week and not the next, so the system will pull orders that are not constrained by lowest priority code and then oldest order date. This is why newer orders and higher priority codes may get scheduled ahead of older orders / lower priority codes.
For example, I have ordered a Platinum on 10/29, and my dealer have me a priority code of 10... haven't gotten scheduled while many XL/XLT/Lariats with higher priority codes and ordered after me have.
The catch is that the deal is able to manually adjust which orders get picked-up, so they could in theory choose to have a new order or higher priority code get scheduled that week. This depends on how actively the dealer is managing their UnScheduled Order Bank (USOB). Priority code + date scheduling is a way for the dealer to not have to manage each week what gets scheduled.