Before you could visit the greengrocer to buy a bag of sugar it was purchased as a solid loaf, cone or lump. Once at home it'd be broken apart with a hammer and chisel, a messy process though with poor precision. The result was certainly not good enough for presentation to fine and important guests.
Houses with higher standards would have not only one of these sugar nippers, but also a set of sugar scissors also. This nipper would first cut the sugar cones or rough pieces chisled from a loaf into imperfect cubes, which would then be trimmed by scissors to create a perfect shape.
The crumbs from this process would be ground in a pestle and mortar for use in the kitchen, whilst the cubes would be placed in a locked tea chest.
You can see the sugar nippers and a sugar cone when you visit the castle, in the pantry.