why class extension?

[m.sniper]

After reading the class extension section, I am very confused of how it benefits the programmer.

I do understand categories, e.g. if I need to add some extra methods to the system class, instead of modifying the system source code I can simply define a new category.

But I just don't see why we need class extensions. One of the requirements of using the class extensions is to define the
@interface existingClass()
-(void) newMethod;
...
@end

and

@implementation existingClass
-(void) newMethod {
...
}
@end

in the main implementation instead of a separate implementation. If so then why don't we just simply change the existingClass.h and .m??

[m.sniper]

nevermind, I missed a paragraph. here is the explanation:

Unnamed categories are useful because their methods are private. So if you need to write a class that has data and methods that can only be used within the class itself, an unnamed category might just fit the bill.