Sometimes in machining you need to make a large hole in a piece of metal. Maybe larger than the drill bits you have around or can fit into your equipment. Also, you might need this hole to be accurate to 0.001 inch. Why I am not sure.
OK, time to deploy a milling attachment called a Boring Head.
It spins around and around shaving off a few thousandths of an inch of metal at a time. Every few minutes you have to stop and adjust it again.
This gets rather tedious. And actually the term "boring" dates from the late 1700s, and is felt to be a figurative extention of "moving slowly and persistently, as a boring tool does".
I can certainly see how a machinist doing this task regularly would have been very happy when CNC milling came along and automated things.