A very sharp blade is essential to mark or 'slash' the top of a risen loaf of bread just before it goes in the oven. The cuts enable a controlled 'burst' of the dough which has two main purposes: it increases the surface area of crust where much of the flavour comes from, and it looks attractive.
Time was when putting your own distinctive mark on the dough that you took along to the communal oven to be baked was an important insurance against being palmed off with someone else's effort by an unscrupulous village baker.
The grignette is a thin scalpel blade set in a plastic handle. It is less inclined to drag the dough than larger blades and is ideal for making intricate markings such as curves and swirls.