What are they used for? I see them used in a few demos/tutorials but I have no idea why people need them. I''m sure there is a good reason though.

You can define face normal (the same normal for each vertex of the face) or vertex normal (one normal for one vertex). It''s useful when you lit an object that is not a simple cube : defining vertex normals is a good way to have a smooth shading and avoid a crappy blocky style.