Journal submission cover letter: length and references
Shouldn't "explaining the purpose of the research and why it's important" be in the paper's abstract and introduction rather than in the cover letter? I'm accustomed to cover letters of the form "Dear [Editor's name], I would like to submit the [enclosed in the old days / attached nowadays] paper, [title of paper], for publication in [name of journal].  Sincerely, [name of author]".