Translation:One uses a cheese slicer in order to cut cheese.
A knife may be the UK "item with equivalent function" for most people, but try an image search for østehovel -- they are not generic slicing implements that would match up with the idea of a "knife" (norsk, "kniv") at all.
Strictly speaking the østehovel is a "cheese plane" and is used with an entirely different motion than a knife. You pull it backward across the top of the cheese, peeling off a thin slice. There is no way to cut with it (in a knife-like manner) by pressing down through the cheese.