"Why can't I just use ==?"

Since it came from C, Objective-C has the same == operator for comparing whether two things are equal.  However, every Objective-C object looks just like a memory address to C.  The two memory addresses will only be equal if the two objects don't just look the same, but actually are the same exact object.

For example:

code:
NSNumber *first_one = [NSNumber numberWithInt:1];

memory:
[--1--]

code:
NSNumber *first_one = [NSNumber numberWithInt:1];
NSNumber *second_one = [NSNumber numberWithInt:1];

memory:
[--1--][--1--]


Here, [first_one isEqual:second_one], but first_one != second_one, since we've made two objects, each occupying a different space in memory.