So that next time someone says "down with tech bros gentrifying the bay area" or they get hauled up in front of the local government who sat the same thing, Google can say "we invested a billion dollars and built more homes than San Francisco added in the last seven years, we're doing our part."

And it sounds like they're 'investing' rather than 'spending' the $1 billion - if you take $750m of land and build flats and sell most of them at market rate, presumably you recoup a fair portion of your $750m - they might even make a profit.