Oldschool planar graphics and why they were a good idea

Video cards of yesteryear didn’t have access to that much ram, so direct color, requiring 3 or 4 bytes of ram per pixel, was out of the question. So to modify the 3rd pixel on the screen, you would modify bit 3 of the first byte of every plane. If we made the 16-color system use planar graphics instead of storing the indices in the low and high nibble of each byte, then going to 32-colors only requires adding a new plane somewhere in RAM.

Source: codeplusplus.blogspot.com