If you live in New York and are applying for a job in California, please explain why (are you moving, looking for a remote position, etc.).
If your work history reflects spending only months at a time on jobs, please explain why (are you a military family, working as a temp, etc.).
If your credentials and work history are far above the skill set for a coder, please explain why.
If there are gaps in your work history, please explain why (stay at home parent, hitchhiked around Europe for a year, etc.).

A cover letter doesn’t have to be long and detailed but it should explain any questions that the resume might prompt.