Dear job applicant,
If all of your training and experience is in a research heavy field related to mine, and you are applying for the job I have posted for an entry-level position in my more applied field, it would be a good idea to write a cover letter explaining why you want to go into my field. Perhaps you have discovered that you would like to apply your love of science to a different type of work. Perhaps you have always wanted to work in my industry, and have only suffered through your more general training with this goal in mind. Perhaps you are desperate for a job and are just applying to anything that seems vaguely relevant. Perhaps you lack basic reading comprehension skills and are completely unaware of the requirements of the job for which you have just applied. Really, your resume doesn't make it clear which possibility is most likely, and I do not have the time to call everyone who applies to painstakingly extract this information.
However, if you are only applying for my job because you can't get a job in your research field, and are willing to settle for my job until you can find the job you really want, you are probably wasting both of our time. I am happy to train someone into this position- my job posting makes that clear, I think- but I am not interested in training you if you are not actually interested in pursuing a career in my field. Or, more accurately, if you can't even be bothered to hide your disdain for my field long enough to get through the job application.
You get bonus points on your cover letter if you point out the things in your background that make you think you can meet the job requirements I posted.
I deduct points if you tell me about your deep interest in your research field and desire to spend your life researching a topic that, while no doubt interesting, has nothing whatsoever to do with the position I posted.
If you don't know much about my applied field and think you might be interested, please go ahead and apply, but this is a case where it is not necessarily wise to be completely honest. You do not need to tell me in your cover letter that you don't actually know whether or not you are interested in my field. You are interested enough to apply, so just write me a cover letter saying that you are interested in my field and highlighting the aspects of your background that make you qualified for my job.
If you can find a connection to me, you can email me and ask for an informational interview (a meeting in which you ask me questions about my field). As I have discussed before, I am happy to do these, and will almost certainly buy you lunch if you are student or a postdoc. I'll be impressed with your initiative, and you'll have a boost over any other applicants, because I will already have met you and gotten to see what a sane, well-spoken, thoughtful person you are.
I will not, however, be impressed by a cover letter that says you are a brilliant jack of all trades who can pick up any field effortlessly. I will doubt that this is true, and think that you are probably an arrogant jackass with whom I do not want to work, let alone manage.
Sincerely, A Very Tired Hiring Manager