My theory behind the move as it has multiple potential outcomes
Hoyer knows the offense better than any of the QBs in the room and can help/mentor all 3 QBs as the team does not have a QB coach
If Mac wins the job, Cam will bolt. That means BB will keep Stid and Hoyer.
If Cam wins the job, Mac is #2 and Stid will be cut thus Hoyer becomes the mentor for Mac
If Hoyer is cut and the 3 others stay, he may retire and stay on as an offensive assistant to mentor Mac and maybe Stid, hell maybe Cam as well.
They also could decide to cut Stid and Hoyer and bring in a better backup QB to slot into #2 in 2022 when Mac becomes #1. Hoyer may stay on as an assistant coach