Shadows and light allow you to give a drawing
some volume. Without that, it can look flat.
In addition, and especially in the manga drawing
style (because it is in black and white), the use
of shading helps to define the appearance of
settings and characters.
It can also offer a different reading of the
scene by creating a certain atmosphere.
By adjusting how much or how little contrast
you include, you can bring tension to an image.
There are many ways to shade a
drawing in a manga style.