If I build a composite on a background that has no obvious existing light direction (either it was photographed in truly diffuse light, or it has no shadow-casting content), I can choose any light direction for my composite content.
Light direction, and especially cast shadow direction are a strong part of a composition. Those shadows ’push’ insistently in a particular, uniform direction. This is a powerful ingredient in the dynamics of a picture. Use it to amplify, not resist the intent of your image.
