It makes the photo stand on its own and shows visually what you were thinking mentally. It says something. The old adage in fiction writing is show don't tell. In photography, the rule is absolute. Think about it for a moment.
Theme requires no special techniques or knowledge, but it does require some thought.
Think about what you are trying to capture. A snapshot is a brief recording of a scene. No thought, no plan, nothing in particular. This is what I saw. On the other hand, a photograph tells a story.