A transparent display shows content while remaining see-through, so whatever sits behind the glass is still visible. In retail it fronts a display case or a window, overlaying price and detail on the actual product.
The reason to use one is that it keeps the physical object as the hero. A customer looking at the real watch, with specifications appearing over it, is a different proposition from a customer looking at a photograph of a watch.
The caveat is that transparency and brightness are in direct conflict. Content on a transparent panel is dim by construction (a large share of the light goes backwards), so the background must be strongly and consistently lit for the overlay to read at all. In a sunlit shop window, which is exactly where these get specified, the effect often disappears entirely by mid-morning.
They are also expensive per square metre, low in resolution relative to cost, and limited in size. This is a hero-fixture technology for one considered placement, not something to roll out across an estate.