Why do CreateObject("roAssociativeArray")?

I am reading some sample code and i notice assoc.arrays commonly created as

aa = CreateObject("roAssociativeArray")

Why so complicated, why not do just

aa = { }

Seems much easier to type and i wonder if i am missing something, as to the reason why it was done (in what seems to me) more clumsy way.

Related to that, why do

aa.posterItems = CreateObject("roArray", 5, true)

and not

aa.PosterItems = [] 

, if for all practical purposes is the same?! (in both cases arrays are re-sizable; "5" is a just a hint about initial alloc, since arr.count()==0)