It is extremely handy to tag people you know in photos you take. You can then easily find images of those people across time and at specific events. This feature appears in Photos in iOS, iPadOS, and macOS, but may not work correctly across devices without iCloud Photos enabled—and sometimes even with it turned on.
Many companies have tried to make this easier by adding facial recognition, so that software algorithms try to guess who is the same person across many faces in your photos, which may include your dearest loved ones and legions of strangers.
Apple’s most recent approach, implemented several years ago in Photos, kept all facial ID local to your computer or mobile device. (As opposed to, ahem, Google.) In 2016, it promised a secure way to sync this information via iCloud, which finally went into effect in Photos 3 in late 2017 for macOS and the Photos app in iOS 11.
