To test application features that require a network connection such as Game Center, Store Kit or simple HTTP requests you need to be able to simulate various real-life conditions as well. You would want to see how your application behaves if the user doesn't have or has an unstable internet connection, or connection suddenly drops during an operation.
Xcode iOS Simulator doesn't provide you such features by default but you can download Hardware IO Tools for Xcode simply by choosing:
Xcode → Open Developer Tool → More Developer Tools
And then login with your account to Apple, download it. Alternatively you can directly navigate to https://developer.apple.com/downloads/ as well. Once downloaded, open Network Link Conditioner and there you are.