As more and more tech jobs became available in SF itself, we started taking them.
I hear Google has an office in the city and, if you do your time in Mountain View and they like you and want to keep you, they'll move you up to the city. I'm sure Apple, FB etc could do the same.
Tranferring to sf is almost always done as a team move, and they will not let you transfer up to the city if your new team does not already ahave a sizeable contingent up there.
It is not a carrot or a reawrd, it mainly has to do with team chemistry and logistics.