Allowing you to login using an id you already have with a major provider saves you time. One can have a 'junk' account with a provider an use it for stuff like

Allowing you to login using an id you already have with a major provider saves you time. One can have a 'junk' account with a provider an use it for stuff like this, if needed. Edit: Having said that, I agree with your premise that a sign in should not be mandatory to using the site.