You go in either when the regular quarterback is injured or ineffective.
People are rooting for you.
If you stink, it’s because you’re a backup and can’t be expected to do what the starter does.
If you’re good, a star is born.
Don Trull was a professional backup quarterback. He played for six years with the Houston Oilers and the Boston Patriots, and every year he was on a card his bio read something like, “Don is being groomed for the number one signal-caller’s job.” He never did get the number one signal-caller’s job — anywhere — but he managed to stay a step ahead of Jacky Lee, who was always being groomed for Trull’s job.
Trull was better than Charlie Napper, who had the best job of all — he was the Packers’ taxi-squad quarterback for what seems like the better part of a decade — but probably not as good as Jack Concannon. The last I saw of Trull was on a set of Canadian Football League stickers, where presumably he gave the Edmonton Eskimos a fine backup who will one day be the number-one quarterback, and one of the top-notch passers in their league.