Effect of bland cover letter for journal submission in biology
All journals I have come across their guide for authors require a cover letter. Although it's probably partially a remnant of older times, where letters and manuscripts were mailed to the journals, it does have functionality.
The cover letter is a quick way for the editor to decide if it's even worth opening the full article. For this reason, arguments on the novelty, findings, importance, challenges and perspectives of the work should be demonstrated. In practice, it might not be much different than the abstract (different structure of course), but some journals also ask for specific questions to be answered.
It also helps the editor, who might not have the same expertise as the manuscript, to decide whether it's worth sending the manuscript to external reviewers. It also saves him a few hours of work from reading the whole article to understand its value.
It also adds a degree of personal communication between the corresponding author and the editor, rather than being a automated, impersonal submission process.
Finally, I usually add sentences like "all authors have agreed to submit the manuscript in this journal" or "no conflict of interest" etc, which adds a signature under the statements.
In the case a cover letter would only say "here it is", I think the editor would probably drop it in the trash bin, unless:
- The title (and the abstract) conveys a subject so important that any editor could not bear not to check it out more thoroughly.
- The corresponding author, or one of the co-authors, is a really big name in the field, that (even if the article is not that important) the editor cannot just reject it.
So, in mathematics, maybe just saying "Here is the solution of the problem X" and everyone knows the problem and die to see the solution, then you don't need to add much. But in sciences where a problem doesn't have a name, a more detailed explanation might be required.
Lastly, the manuscript is indeed evaluated by the manuscript itself. However, a good cover letter might make the editor check your submission more thoroughly, out of the tens or hundreds of manuscripts they may receive.
I think of it as the trailer to a movie. If you like Batman and you see Batman on the title, you will check the movie a little more before deciding if it's a crap movie or not. If you see a movie is directed by let's say Spielberg (and you like him), you may do the same. But if you are unsure about the title and you don't know the actors and directors, a trailer might convince you to watch the movie.
Mathematics and Biology have polar opposite cultures, and much of this is due to what's practical. Some of the top Biology journals receive tens of thousands of manuscript submissions each year. For example, Nature receives 200+ manuscripts each week. They send maybe 30 or so of those out for peer-review. They can't read 200 papers in a week, and likely wouldn't understand them anyway. So they must assess their significance without reading the paper. The cover letter is written so that any reasonably well-educated lay-person can understand why the paper is important, even though the paper itself (even the abstract) maybe too technical. This is actually a somewhat reasonable test to show how important the paper is. It's really hard to sell the importance of an obscure unimportant topic in layman's terms.
As one moves down the food chain of journals (e.g. journals with lower impact factors, that are eventually accepting 25+% of what gets sent to them), the cover letter becomes less important. The journals are less broad, and the editor can assess the importance of the paper from the abstract.
In addition, there is just a culture in mathematics of being slow and methodical about peer-review. Editors actually read the paper. Reviewers will spend a year checking your proofs to make sure they are correct. So the added time it takes the editor to read your paper is epsilon compared to the review process. I'm a mathematical biologist, so I get asked to review both applied math papers and biology papers. The math journals usually give me a deadline of 2-3 months to get my review back, the biology journals usually give me 1-2 weeks. Biologist reviewers aren't repeating your experiments, like mathematicians are checking between the lines of every sentence in your proof [or scowering their brain for counterexamples]. So if in biology it takes 1-2 weeks for the reviewers to get their reports in, the time it would take the editor in chief (EIC) to read every submitted paper would be a significant percentage of the review process, and would really slow things down.
The cover letter is for statements like "We performed the first-ever controlled experiment validating ..." or "We perform the first explicit assessment of the accuracy of method X from paper Y (Y has been cited 1,390 times since 2015, 382 of those citations are from papers in Journal Z [the journal you are submitting to]) and find that 82% of papers in the field are using the method inappropriately."
These are the types of sentences that would be extremely weird to write in a paper or abstract but would be useful information for an editor to have. As the journal becomes more specific, the cover letter is generally more about why the paper fits well in that particular journal, rather than why its the greatest paper on earth (which might be what you'd say if you were submitting to science or nature).
To answer your question about a bland cover letter, like "here it is". While this would likely lead to an immediate rejection at a journal like nature or science or PNAS, at more subject-specific journals it might not end up hurting the paper's chances. The cover letter is to point out things that may not be obvious to the editor in the abstract. Things about citations to papers you are building off of, some more tenuous implications of your work that are too uncertain or controversial to say in the paper, how your paper relates to other papers published by that journal. If you think the importance is clear from the abstract, and you don't have anything additional to add, the cover letter isn't all that important, even in biology. I know several handling editors who say they never read cover letters (although presumably, the EICs do partly to select the appropriate handling editor).