I remember this from Win 95 and Win 98, maybe even XP? You usually had a GPU, but they were more simple than today’s GPUs. Oftentimes they were on-board (in contrast to today’s iGPUs that are part of the processor, not part of the mainboard).

In the very early times you had 3D cards like the 3Dfx Vodoo cards. Those could only render 3D. Your desktop and other programs were still rendered by your normal GPU or on-board graphics card. They did only render 3D. You had to put a small cable from the output of your regular GPU to your 3D card’s input and then plug you monitor into the 3D card. While you didn’t do 3D, the 3Dfx Vodoo would just output everything it received from your regular GPU. When you played a game, it would output whatever it rendered instead.