Shading is valuable to realistic drawing, because real life has shadows and lighting everywhere. In order to mimic real life, you first need to utilize the aspects that make objects in real life, look 3d. In my opinion, you can get batter at value drawing by shading and highlighting simple 3d shapes. Like shading a circle into a sphere, or a square into a cube. Highlighting simple 3d shapes on a black paper can also help you with drawing value. Practicing shading and highlighting in general can help you get better at value drawing.
