Do not need to confirm your e-mail or fill out a long form. Your basic information is already register at social networks, we only get that information and done! You are already registered or loged into store.
In addition to being more practical, social login is more secure.
Use the same password at several websites is too dangerous. On the other hand, it's difficult to choose a different password for each site and then remember them. So, the login through social network, with OAuth 2.0 protocol, is more secure and efficient.