What is the second parameter of NSLocalizedString()?

What is the *comment parameter in:

NSString *NSLocalizedString(NSString *key, NSString *comment)

If I do this:

NSLocalizedString(@"Hello_World_Key", @"Hello World")

and have two versions of a Localizable.strings (English and Spanish), does each need the entry:

English.lproj/Localization.strings: @"Hello_World_Key" = @"Hello World";

Spanish.lproj/Localization.strings: @"Hello_World_Key" = @"Hola Mundo";

Isn't the English one redundant?