Signing in gives access to user data. In return there should be some utility to the user. If there is no implication of the latter, the user won't give away the former.
It gives access to your name, email and profile picture (if you have one). I don't consider it any different than if the same was asked with a signup form.
Having implemented Google Logins in the past, I always choose that option on sites I see: The site doesn't need to store any kind of password for you. This site also appears to be using the new Google "Cross Account Protection" feature, which will for example automatically log you out of the third party site if you report to Google that your account is compromised.
Having implemented Google Logins in the past, I always choose that option on sites I see: The site doesn't need to store any kind of password for you. This site also appears to be using the new Google "Cross Account Protection" feature, which will for example automatically log you out of the third party site if you report to Google that your account is compromised.