I can see the potential of collecting and aggregating large amounts of data on individuals, but only for selling as datasets to other businesses - for example I am sure health insurance companies would love to see fitbit data to help them increase your premiums.
However there must be a limit to this from an advertising perspective. Even if my best friend was deciding what adverts I should see online, I would be very surprised if I would suddenly start buying more stuff that I don't need.
Maybe I am looking at this the wrong way. It could be that companies that want to advertise simply don't understand that after a certain level of targeting, your revenue will not increase, but your costs will. If companies (and political parties) continue to demand ever more detailed targeting capability, then Google is simply meeting that demand.