So for anyone looking into old school graphics programming, bit planes are pretty confusing when you don't understand why they exist.

Two big reasons. First, it's about running memory chips in parallel to increase bandwidth. Image data was hard to get to the screen fast enough with hardware in that era.

Second it allowed for simple backwards compatibility. Programs were used to writing directly to video memory, and in an EGA card the start of the video memory was valid CGA data. The rest of the colour data was in a separate bit plane.