As a backup. I'm sure Cam is thrilled. They cut Cam so they don't have to pay him.
They might've offered him the backup job and maybe he declined. I don't know, but it's better for Mac to not have a distraction. We kinda knew this was a possibility when more details emerged on Cam's 1 year deal and saw how likely it was for him to get cut and the Pats not be too hurt by it.
Good news for the Pats is that if Jones is a total bust, they can get Jimmy G back next year or even later this seaosn if Lance replaces him.