I think it exists because it's a concise way to communicate "this is ending, and will end in a predetermined amount of time, but it has not yet ended" in the same manner that sunset indicates that a day is ending but has not ended yet.
A Google+ private community allows social, collaborative photo sharing photos with friends with login required. This is not supported in Google Photos! With Photos shared albums you can select people with whom to share, but those people just get an email with a secret token embedded in a URL -- aside from trusting your non-technical friends not to accidentally leak this URL, there is no way to know who is viewing the photos or remove access from specific individuals. I really hope the Photos team upgrades their shared album permission model now that the alternative in Google+ has been sunsetted.