Are cover letters sent to referees and should you highlight the contribution of your paper in the cover letter?
It is also polite to write a cover letter, even if nobody asks for it. And the purpose is often to ease the process of finding the referees, so helping to do so by giving insights about the paper that cannot be write in the paper is a good thing.
If you mention potential referees, I guess it will not be sent to the chosen referees. Otherwise (in my experience) it is.
You don't have to praise your results in the cover letter. Just state shortly the results (say more than the abstract, less than the introduction). If you think that there are issues related to the paper, this is also the place to state them (conflict of interest, new way of considering something - which can hurt someone's feelings -, direct negative comment on related work, etc.). If you think that, because your paper is gap-bridging, you need reviewers from several fields, you should say it as well.