Managing digital photos is quite hard to do reliably.
Where do you store them? Optical disc, it might get mushrooms; HDD, mechanism might fail; SSD or flash, this one’s better but it might get corrupted, and so on.
Cloud services provide a convenient solution to all this, than apart from the service going down (which is less likely) have no other issues. You can also access them wherever you are.
Privacy is an important concern. It would be nice to have them encrypted on cloud. Encrypted from a local and trusted (open source) client, that is also convenient. If each time I want to show a photo to my granny I have to download and gpg a file manually, I pass.
But most people don’t care about their privacy at all anyways, so why bother.