Imagine a Mac Pro configuration that is a 3U wide (high) and comes with a tower stand base and handles on top that are installed at the factory. Single power supply and a state of the art GPU.
No imagine the same base system, but instead of a tower base and handles, they are replaced with rails and each mounting hardware. Instead of a single power supply, it comes with a dual PSU (such as this) and instead of a state of the art GPU, you get a bare bones PCI-E card with a mini DP port with integrated LOM, so the card would also have a dedicated ethernet port for it. Presto, a suitable machine intended for the server room.