That declaration makes sure that the UIView returned conforms to the NIPagingScrollViewPage protocol. The compiler will emit a warning if the method tries to return an object that isn't declared to conform.
A more common usage of that syntax would be a delegate, as you'll allow any class that conforms to the protocol to be the delegate, so that syntax is used to make sure the class conforms to the protocol.
-(void)setDelegate:(id<SampleDelegate>)del //Makes sure that del conforms to the protocol SampleDelegate, the compiler will emit a warning