I remember during the early McCarthy era, Young Understudy Rodgers was tasked with giving the CB report to the QB room and live-charting what he would have done on every one of Brett's snaps.
I never thought about the backup having more responsibilities than getting donuts and giving nods when the coaches say, "Did you catch that?" But giving the young guy some responsibilities (at a level that he can handle) and making him face the entire team is good prep that this isn't just about getting a game check and asking the coach when he can come in.