This model is meant to ssecure a PCIe card to a plat surface, then the motherboard is screwed onto same flat surface. You can use say a plywood plank, secure the motherboard on top, and this to the PCIe card and to the edge of the board.

There are two main versions : low profile and normal profile.

For ease of printing, you print the plate in the size you want, then the bracket, and screw both together. The holes where the PCIe backplate screws onto are meant to recieve threaded inserts.

You can see in the pictures the use case, and how to use it.
It is far from perfect, but it gets the job done.

You can use it to make some kind of open air bench, with a piece of plywood and some 3D prints.
 

In order to assemble a PC, you need some parts : 

• A “case”, it may be as simple as a plywood plank of suitable dimensions (up to you);

• A PSU plate to secure the PSU in a similar context (by me);

• A PCIe bracket support to secure PCIe cards in a similar fashion (by me);

• A motherboard support, by DockGuy;