Is that a thing "Anything you upload to our site belongs to us" I was wondering about that myself for my own services. If I can get people's data, then you could use that to train ML with and then your problem of lack of data is solved haha... but this is beyond me at this point just thinking out loud.
Photos contain private items, things like clothes. So they allow Google (and Apple, and MS, and Amazon) to construct "real-world" metrics to brands and vendors about which clothes they bought, maybe even with odds as to where they were bought, and maybe base advertising decisions on that.