Anyway you can use data to nudge users. For example, Google can change search result orders. They can promote one company/research/ideology/party to the top and demote others.
This is advertising.
Finding out where certain people are important for law enforcement or press.
This service isn't that valuable, and extracting the value required is going to be a PR nightmare.
Stores give out free wifi to track your MAC address and see where you go in stores. They sell this data, use it to track theives, or use it for better product placement.