Specifically, the Google parcel layer is there to build up the foundation for the sale of hyperlocal advertising. So says WeoGeo's Paul Bissett, and I agree with him.

Since the appearance of parcels on Google maps eight days ago, an army of GIS experts derided Google on multiple Internet forums for seemingly biting off more than they could chew and getting into a business they do not understand (How current is the data? What is the accuracy? What is the source? How about maintenance?). Bissett’s hypothesis puts things in a different perspective, and posits that for Google, the parcel polygons do not represent property boundaries, only a much better approximation for street addresses than Google previously had.

Reasons Bissett:

and

This all makes sense. For the record, I still maintain that Google will turn the parcels off in the near future (while continuing to use them for routing, etc.), for the same reason they now do not allow users to view parcels over the aerials -- too much confusion, too many people will jump to the wrong conclusions.

Full article.