IMO - this is all just my feeling - Cam is a popular QB who speaks highly of the culture/program in NE and will help with signing the offensive pieces they want (Samuel, etc). They still want Garoppolo if he can be had and know they will have to wait a bit. If NE makes a play later, Cam will become an insurance policy against Jimmy injuries but will also see the field in certain packages and/or games Jimmy sits due to injury.
Cam may have a better shot at a bigger contract next year if he looks decent in limited duty, vs. another year starting where his weaknesses continue to show.
The downside to that approach is you're committing almost $30M in cap space to QBs so don't have a lot of space to work with for other pieces. They may have to work something out with Jimmy at that point, contractually. They'll have some leverage since, considering the cap situation this year, most teams will have already spent what little cap space they had, so there won't be many mathematically possible suitors.
I think if they get to draft day and SF hasn't made a move for a QB, they grab a guy like Lance and forgo a move for Jimmy entirely, at least this year.