"I'm looking through you..." so went the chorus line of a Beatles song. And it definitely applies in terms of the effect one is after when implementing a rear projection in the retail space.
Retail display is all about attraction, intrigue, hipness, enticement. It has to make you want the product. It has to promise more if you'll just come in the store.
That is why rear projection, and in particular, window video systems, are such a great starting point, and such a potentially powerful display tool. But once you decide it would be cool to have a moving image directly on your storefront window, then what? Do you need to have a solid, high-contrast image? Or would it be better to actually be able to see through the image?
Allowing for a digital image directly on the window glass, in essence, a digital window sign that can be seen through, could be the exact effect necessary, where there is a combination of actual product displayed, combined with the Digital Window effect.
Holographic means you have a sort of "ghostly", or see-through effect to the image. Granted, you lose some density with this approach, and you lose some viewing angle, but you gain a very distinct, subtle and surprising effect of having "animated" window glass.
This is markedly different from a more dense, high-contrast images requiring high-contrast window treatment that renders the glass opaque (usually charcoal gray.)
Holographics = surprise, intrigue and coolness. It's the "whoa, how'd they do that!!!" effect. And that can be a very powerful ally in point of purchase display. Now you've got them looking.