Excellent point. There may be another angle. JG has been with the Rams 2 years. He knows the playbook. He knows the RBs, TEs and WRs. Maybe it's a little frustrating to take snaps in camp on Stafford's day off, work your butt off, knowing full well, you ain't playing come September.
FWIW, I think it's more about starting. He's 35. If somebody around the NFL goes down, he may be getting a call to back up somebody like Fields. Or Trubisky. Or Pickett. You know, somebody who could start a game or two and get benched.