Your computers motherboard is the large circuit board which all other components connect through, it is responsible for handling communication between all internal components.
Like CPUs, motherboards are constantly evolving, a new CPU release often means a newer / faster motherboard is also released to support the new processors.
For Intel based Enthusiast CPU computers the fastest motherboard chipsets are the X299 boards.
We change the motherboard used in a computer build automatically based on your selected graphics setup. This is due to different motherboards having different options on them for hardware support.
The motherboard upgrade cost is included with the graphics card upgrade price.