Why should you write a cover letter when submitting your article?
A cover letter is a personal letter to the editor in which the authors' have the opportunity to explain why you believe that the manuscript has an important message for the readers of this specific journal. It is a kind of a sales-pitch which is very important, but of course it should not be exaggerated.
It is impossible as an editor to grasp all subjects within the journal’s area. So it is quite all right that you with simple words explain that the findings are important and why it would be a good idea to publish the article in the journal.
These cover letters are actually read by the editors so it is valuable for you as an author to spend time to write a good text.