The purpose of a bread lame is to slash - or score - the surface of the dough before it's placed in the oven for cooking. This technique allows the bread to bake evenly without the surface or crust cracking in unexpected places, while preventing those large pockets of air forming in the loaf.
Scoring the dough expands the surface area of the crust of the bread, allowing carbon dioxide to be released from the loaf during the baking process which contributes to the holding of its shape. It also means more of that wonderful, rich-tasting crust for everyone to enjoy.
These tools allow you to control the direction of expansion by gently score the surface prior to cooking which not only improves the flavour, but also creates beautiful, unique designs to ensure your end product looks just as good as it tastes.