Using shadows in a painting can affect the way the subject is viewed. The temperature is altered by their colour, blue creates a cooler effect. Warmer colours such as grey or purple are used to create the suggestion of shadows on a warm day. Deep shade throws the subject into near darkness against bright lighting; lighter ones provide a softer effect.
Atmosphere is created by the use of shadows - threatening by deeper tones; soft shade by lighter ones. They add to the structure of painting by casting off in different directions. They need to be believable, that is go in the right direction from the light, but add texture and depth to a simple form. Only abstract paintings allow for the wrong direction.
Without shadows or shading, a painting can be flat and detract from its interest. The slightest light present forms a shadow. Don't omit it from the finished work.