First of all, check the submission guidelines for the journal that you want to submit to. If there is any information regarding cover letters to editors for manuscript submission, this will overwrite everything I say here.
Second, disciplines vary widely in their conventions regarding cover letters and in some it is in fact considered bad taste to write one. If you don’t know what the standard in your field and there is no information on the journal page, ask senior colleagues or email the journal or editorial office if you should submit one or not. (Be ready to receive an arrogant reply because people are often ignorant of the fact that cultures and conventions can differ substantially but by no means take it personal.)
If there are no clear guidelines against a cover letter and you have not encountered horrified faces/email replies when asking around, just write it. Even if it is not obligatory, just do it. The goal of this letter is to make the editor’s job easier (remember these are overworked academics who do this as a side job) and this ultimately can have a positive effect on how and how fast your submission will be processed. Here are tips how I do it (template and mini-summary below):
What I need you to take home from this:
- Know and communicate what article type you are submitting and make sure it fits ALL requirements in the guidelines for authors!!!!
- be concise and professional, jokes are strictly prohibited (even if you are best friends with the editor!)
- do not oversell your research
- avoid jargon and name dropping
- clearly communicate why your article should be published in this journal
- make sure you include all legally required statements (Guide for authors!)