Cam can be starter and Hoyer can be the guy to get you through a game should Cam go down. If Cam goes down then you can decide to spend the entire next week prep'ing one of the young guys and use Hoyer as his backup. Or if Cam stinks he can be cut at will with three other viable candidates to step in.
Interestingly he has started 39 games for 7 different teams and was named the Browns starter at the beginning of 2013 and started the first 13 games, so I'd say he was more than a career backup, although he did not earn a long term starter's job. It does help explain his career earnings.
en.wikipedia.org
Kinda has some trippy stuff, like him starting games for ARI and IND.
BPA, baby!