[sahaskatta]

It adds it to Google+ Photos. It consumes the space which Google Drive provides, but you can't browse the photos from Google Drive. (Side Note: If you upload below a resolution of 2048px, it doesn't count against your storage space.)

Also, Google+ offered this for every Android device long before Apple's iCloud backup, Dropbox Uploads, or even Microsoft's OneDrive auto-backup.

Since most people get confused and avoid the feature, I think it makes sense for them to create a separate product called Google Photos which has nothing to do with social.

[cookiecaper]

Auto Upload definitely sounds very scary the first time you're prompted about it. Most people fear that every photo they take will automatically be publicly shared on Google+ (which isn't the case; it's uploaded to Google+ Photos as a private document that "only you" can see) and go out of their way to make sure it's off.

I've found the feature convenient and look at most of my pictures that way. It's a bummer when I have to connect my phone via USB cable and pull stuff off that way now (especially since Android is now MTP access only, but let's not go down that road...).

[balladeer]

But then they might want you to login only with a G+ profile if at all they let people comment or so it will require them to use their Google+ photos.

Or if someone wants to have a Google Photos URL to share, Google will give an option of "photos.google.com/<one fucking long ugly random URL>" or "photos.google.com/FirstnameLastname<add some numbers or characters because others may want this URL too!>". That's just ugly!