Why call Primitive Geometry 'brushes'? Why BSP?

Why on Earth would someone create a complex level with Primitive Geometry, then waste time doing it all again? Creating the same exact thing twice… WHY? Why would anyone ever waste time and do something twice? Why call, GEOMETRY ‘BRUSHES’?

Legacy reasons, it’s pretty much archaic, but can still an important tool to old school level designers. UnrealED and Valve’s Hammer both refer to those geometry editing tools as brushes. I’ve always assumed it’s considered a brush because you are adding and subtracting at a certain area in space.