[AnthonyMouse]

If customers know your product exists before they can buy it then they may wait for it. If they buy the competitor's product today because they don't know your product will exist until the day they can buy it then you lose the sale.

Samples of new products also have to go out to third party developers and reviewers ahead of time so that third party support is ready for launch day and that stuff is going to leak to competitors anyway so there's little point in not making it public.