But many others sign up for these services without ever quite realizing what they've given away.
For the rest, I suspect Google is inventing its own supplemental metadata, using rapid image scans and automatic face detection as part of the company's continuous "machine learning" system.
It may not be perfect at the start, but as Google gets more and more photographs to scan, it could become the most accurate auto-tagging service on the Internet.
In an interview with Medium's Steven Levy, Google Vice President of Streams, Photos, and Sharing Bradley Horowitz talks a bit about their systems for doing this: We have a proliferation of devices and storage and bandwidth, to the point where every single moment of our life can be saved and recorded.
When you offer a free service, you're likely to get many more people to sign up or download it than if that service was offered at a cost.
In plain English, that means being able to sell custom ads against your data: showing you a Nordstrom ad for shoes, for instance, because Google knows from search or web history that you looked at some yesterday, or from Gmail that you purchased heels at the store last week.
The company uses this data to sell to advertisers, and advertisers in turn get a much closer look at you, your spending habits, and your daily activities.
For some people, this is a perfectly reasonable tradeoff for "free".
Google Photos storage won't cost money, but you should know what you're signing up for. You can automatically search for faces, places, and things without tagging! And, oh yeah — unlimited* (if your photos are 16MB or less) storage for . Well, to start, it's important to know what "free" might actually cost you, and whether you're okay with that tradeoff. As a result, you get an awesome service you don't have to pay money for. But because the company isn't charging for its services to get those user numbers, it still has to make money — it isn't a charity.
Updated on 5/29 10AM ET to add information from Steven Levy's excellent interview with Google Vice President of Streams, Photos, and Sharing Bradley Horowitz, and to add information about Google's high-quality photo pricing tiers. As a company, Google has a lengthy history of offering free services with outrageously awesome storage capacities: Look at Gmail or Google Drive. And therein comes the potential dark side of Google's data usage, where the company pays its debts by leveraging its biggest asset: you.