Watch with transparent case back

 

— a watch with a case back made of transparent materials, so you can see the mechanism without opening the case.

This is purely a design decision that does not affect functionality. It is used in models designed for connoisseurs of "watch aesthetics" — mostly mechanical, but there are also quartz ones. Note that most watches with a Skeleton-type case (see the relevant paragraph) have this feature: the combination of such a case with a transparent cover allows you to see the movement both from the front and from the back. However, such a cover itself is used much more often than "skeletons" — it is simpler in design and can be provided even in relatively inexpensive watches.