Installing nvidia-driver removes xorg

[Gregory Sharp <gregsharp.geo@yahoo.com>]

I am currently running stretch with the nvidia display driver.  The
nvidia-driver package is not installed.  Instead, the nvidia-kernel-dkms
package and other packages needed for the display driver and CUDA are
installed. 

While attempting an upgrade to CUDA 9 in backport, I learned that installing 
nvidia-driver (either version: 384.130 or 390.87) would uninstall xorg, gdm3,
libreoffice, and many other seemingly useful packages.

Questions:

1) Why should xorg be uninstalled when nvidia-driver is installed?

2) What might be a recommended upgrade path for me?