A website can have a number of options in terms of user management. Authentication is a difficult problem to get right and sometimes a website might delagate that problem to someone who has it nailed down. Google lets sites use their authentication service (typically underpinned by a protocol called Oauth), for other sites to manage their users. Other times a site might let users have the option of storing their account on their servers or Google's servers. For me, it's convenient to use my generic google account when arbitrarily logging into a site that requires an account to use it.