I suspect it's a way to dither with few colors, and possibly a way to render textures very quickly into the Amigas peculiar pixel format. One pixel is not a triplet or bytes, or even a single byte. One byte is 8 pixels!

And that's for monochrome. (Pure black and pure white.)

To have more colors, you add more of these "bit planes" on top of each other.

So if you have say, 32 colors and want to update a single pixel, you have to write four bytes, changing only the one bit in every one of these four bytes.

This is not very nice when you try to sweep textures on a 3D-ish mesh.

(But very good for 2D graphics with large chunks bobbing up and down.)