paying that kind of cash for Zach to compete with a kid that nobody's ever heard of for 4th string (which means practice squad, which means Zach would have been cut). If Zach gets cut for DiNucci, his NFL career is pretty much over.
Zach is being brought in to compete with Stidham for the placeholding starter role. They know they will be drafting a QB, but they also know they pick 12th . Whomever they draft is not likely to be their opening day starter. They want either Stidham or Zach to take that role and hold it down until they can bring their rookie along.
If at any point Zach finds himself competing with DiNucci, he might as well start planning his post-NFL career.