Google doesn't share your photos with the web automatically. It may have upload them, but they are not automatically shared with anyone. It maybe that you, or another member of the household installed Google Photo backup. Google Photo backup could have come bundled with another product you installed.
You control what is shared and what is not.
As a prolific google photo user, I have 30,000 pictures, and not a single one was shared without me clicking the share button.
For me, it provides a convenient way of backing up my photos in case of fire,flood, theft, hardware failure, and etc.