Cover letter for journal submission (math)

Unless the journal asks for any specific pieces of information, then a "Here it is for your consideration" is all that's required. In the (math) journals I'm an editor of, that is essentially what the letters look like, and they almost never contain anything that is of importance to the editor or to reviewers.

I believe that this is very different in other areas, but then the submission guidelines will say so.

My impression is this often only gets asked because the submission system was set up for general journals. I agree with Wolfgang's answer that you do not need to include much in the way of content, but will add that one thing you can do is use this letter to suggest possible editors for your paper (sometimes there is no other place to put this). It of course makes things easier for the editor if they get papers in areas they know well.