class-extensions

I don't understand why programmers use the extension keyword in their class implementation. You can read in other topics that code is then more semantically separated and etc. But when I work with my own code, it feels clearer to me to use // MARK - Something . Then when you use methods list (ctrl+6) in Xcode, everything is seen at first look. In Apple documentation you can read: “Extensions add new functionality to an existing class, structure, or enumeration type.” So why not write my own code directly inside my own class? Unlike when I want to extend functionality of some foreign class,

Class extensions @interface Class () are a lot more powerful and can inject variables into the class. Categories @interface Class (Category) can't. What other differences are there, and when should one use a category over a class extension? The main difference is that with an extension, the compiler will expect you to implement the methods within your main @implementation , whereas with a category you have a separate @implementation block. So you should pretty much only use an extension at the top of your main .m file (the only place you should care about ivars, incidentally) -- it's meant to