A CPU is mostly good at doing linear tasks very fast, and a GPU is mostly good at doing many things at the same time.
Games involve the drawing and animation of a lot of 3D models and textures, which is where the GPU really gets to shine.
A better GPU has more processor cores, and processor cores that are faster, allowing it to draw more things more easily, which allows a game to look better and run at higher framerates which make it look smoother.
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