The install process includes the option to sign up to Rockstar's Social Club as well, which is necessary for online play, and performs an online security check to ensure that you are a legitimate customer and therefore qualify to be treated like a criminal.
That phrase is a little unclear– is it still necessary to sign up for and install Social Club if you don't play MP? If I ever pick up the PC version it'll be for SP only, so NOT having to use SC would be welcome. But then it sounds like it might be necessary for the online check, even if you're not using MP. Can someone clarify that?