Before we answer the question of why we need to connect to a graphics card, let us first understand what a graphics card is.
A graphics card (also called a video card, display card, graphics adapter, video adapter, or display adapter) is an expansion card that generates a feed of output images to a display device (such as a computer monitor). (Source: Wikipedia: Graphics card) It is used to convert data in your computer to useful images on your monitor. This can either be integrated or discrete (or sometimes called dedicated).
An integrated graphics card is embedded in the CPU or motherboard and not a separate card. A discrete or dedicated graphics card, on the other hand, is a distinct chip or board that is separately installed in the computer. For desktop computers, this is usually found in the middle area at the back of the CPU. Due to the demand for higher graphic quality these days, most desktop computers have a discrete or dedicated graphics card.
So why do we need to connect to the graphics card? Most often than not, when a discrete or dedicated graphics card is installed on your computer, the display connection gets routed to the graphics card so the display port on your motherboard will no longer function. Sometimes, display ports (HDMI, VGA, DVI, DP, and/or USB-C) on your motherboard can still handle display connection but oftentimes, they won't and the external display you connect will either show "no signal" or have a black display. Thus, it is recommended to connect your Huion Tablet Display to your graphics card and not to your motherboard.
How to determine if your computer has a discrete or dedicated graphics card and which set of display ports is the graphics card? If you see 2 sets of display ports (HDMI, VGA, DVI, DP, and/or USB-C), it means you have a discrete or dedicated graphics card. Find the set of display ports that are separate from the other non-display types such as USB-A, audio, and LAN ports. The set without these non-display ports is the graphics card and this is where you need to connect your tablet display.