>What information is the CPU sending to the GPU that it can't just send to a display

It's a bit like this image. Very much simplified, you can think of the CPU sending information like this

• There are 3 vertices (points) at the x, y, z coordinates (340, 239, 485), (312, 285, 512), (352, 297, 482) that form a triangle.
• The vertices have these and those colors, textures, bump maps, reflective values, opacities etc.
• The camera looks at them from position (112, 756, 912) with this and that angle, viewport, zoom.
• There is a spotlight at (567, 88, 45) with this angle, shape, color, intensity. There is another one at (342, 1274, 1056).

And the GPU will come up with

• What is the RGB color of pixel 1234, 342 on the display.

As others have answered, the CPU could do this, but the CPU is optimized for doing a bit of everything, and the GPU is optimized for doing a lot of floating point (decimal value) calculations in parallel.