I believe it is just another measure to keep it exclusive and ultimately, I think it works. If you want something, you kind of "prove" it by actually calling or stopping by, etc... Online is pretty impersonal and it's also easier for people to steal the designs. As it is, there are only a few pieces online and I think that with the way CHANEL does it, the pieces are less likely to be copied and only people who are willing to put a bit more effort in to finding them end up getting them, rather than the masses.
At least that is my take on it.