Photography is all about light, literally.
A photographer learns to see light and how the camera will reproduce it. After years of experience they can predict light; when and where it will be best for a particular photograph. Experienced photographers understand the quality of light and all its variations: soft, hard, warm, cool, direction, etc.
The best photographers control light. They select, create and modify light(s) to evoke an emotion, a response from the viewer.
A professional photographer has an investment in techniques, knowledge and experience to deliberately create what a client needs – on demand. Professionals will have the resources in terms of techniques and tools to respond to changes, whether that’s a client request or sudden change in circumstances. They can’t rely on happenstance but can take advantage of the ephemeral nature of light as it changes.