Semitransparencies as a tool and design element
It's a common practice to overlay objects in charts to show they span the depth and/or width of other objects. it's less common to make those objects semitransparent. It's a nice technique that adds a little more depth to the concept being communicated. Take, for example, the image below.
The squares in the matrix are like more like a stained glass window than a flat, opaque matrix. Both are very usable, but the semitransparency of this graphic is a little less used and very pretty.