Hi, thanks for the course Kent,
Afterwards I experimented with using different textures to create the base sand dune shape (e.g., wave, gradient) and found that in each case the effect of the texture is not only to displace the height of the plane in the z direction, but in the x and y direction as well. The voronoi appears to do the same (try turning the multiplier up to 50). You can fix it by adding a displacement node, but I'm not sure why. I'm guessing it's because the black and white texture effects all three channels of the displacement but if you plug it into the height input of the displacement node first it restricts it to just the z channel (?)
You're hitting on something similar to what's been discussed in this thread. I noticed too that the procedural displacement doesn't seem restricted to vertical (or pure face normal) direction. I don't see the effect that strong in your image but I did notice it could get pretty strong when I played with the material in my scene. This ability is surprising but I'm glad it's possible because it offers more interesting results depending on the situation.
I don't know exactly why it functions this way since I also expected it to only work in the pure normal direction. Though I'm glad I now know that the displacement node negates this. Someone else was asking about the displace node and why I didn't use it. Now I have a reason :)