Well, mostly, alloc init, is for creating the memory part where it's gonna put it, like every other object (nearly). there is a biiig explenation about this, i'm sure some of these guys can exactly tell you, it's something with bits on the heap stack, and blah x) But it all comes down to memory management.
Also, a lot of times there are extensions to init (for example, initWithString:) but no corresponding extension to new.