Historically, you'd mark up a drawing or schematic with a grease pencil (1) on top of a velum sheet or similar. The original implementation of grease pencil in Blender was similar -- it was for non-permanent markup on top of your scene. This gradually evolved into what, in Blender 2.8, is a full on drawing tool that can be used in part to create (with Blender's other tools) entire 2-D animated features. I agree they should change the name soon. :)

1. https://en.wikipedia.org/wiki/Grease_pencil

It's from before, when it was a tool for marking up the scene, not an artistic tool. It's retained the name since then.