These pieces are for a Geeetech aluminum prusa i3. The purpose is to secure the base of the printer to a platform, using the printer-to-base clamp part, and then make the z-axis perfectly square with this platform while reducing any wobbling of that axis by using the square mount for z axis part (x2).
Since the z-axis only contacts the y-bed/axis at two on the very bottom it causes the z-axis to lean forward due to both of its motors to be hanging off of it vertical face. Also, once the x-axis is installed it causes an even greater lean.
For my platform I used three pieces of 20″ x 20″ x 1/4″ MDF sheets, which i screwed together. Upon centering the printer on the platform I secured it using 4 x printer-to-base clamp (2 at either end of the y-axis). Then the two square mount for z-axis pieces could be fastened the the platform and printer.