Vertex normals are used in Gouraud shading, Phong shading and other lighting models. Using vertex normals, much smoother shading than flat shading can be achieved; however, without some modifications to topology such a support loops, it cannot produce a sharper edge.
The use of vertex normals in shading techniques like Gouraud and Phong shading allows for smoother shading compared to flat shading.  However, achieving sharp edges requires additional adjustments to the mesh topology, such as support loops.