A course in relation to photography helps a photographer regardless of his/her level of proficiency to understand a certain technique or theory or process or rule in photography. Needless to say, there are various types of photography courses, each aimed at a different level of proficiency.
A photography course helps in fine tuning and brushing up the knowledge that even a pro photographer may have already acquired. Thus, there are courses which teach advance compositional guidelines, shooting techniques, use of advanced photography tools and post-processing techniques to professionals.
At the same time, there are courses which are geared towards the basics of photographic composition, an understanding of light and how to choose photography tools such as camera bodies, DSLR lenses, and lights which are aimed at beginners.
A photography course need not always have to be attended in person. These days several online photography courses help photographers to learn from the comfort of their home.