The subject element is used to describe the subject content of a digital resource—what’s in the photo, what the picture is of and sometimes what it is about. (There have been a few blog posts discussing ofness and aboutness.) It’s the topic of the content. It is also used to facilitate searching and browsing a digital collection by subject and collocating resources with the same subject content.
So, not only does the subject element describe the content of an image, it is also important in a descriptive metadata schema because of its role in the functionality of searching a digital collection. When users are seeking content in a digital repository, they will likely do exploratory searching or browsing by subject.
This is no less the case for our class indexing project. The subject element would be useful in the football photos to aid searching for a particular player, play, or game. It would be useful for the DePol art prints to identify a particular print and the content of a print.
I would argue it is one of the most important metadata elements because of the information it provides to the user and its role in the function of discovery and retrieval in a digital interface / database setup.