If you are writing to a liberal-arts college and you attended one, say so. If you're applying for an interdisciplinary position and your academic training is interdisciplinary, say so. If your work could be an especially good fit with that of the current faculty, say so. And if you are applying to colleges in a specific geographic area that is not generally popular, but appeals to you, let the committee know that you want to work there. If you want to give a one-sentence summary of your interests and your qualifications, that's fine.
People commonly offer to provide additional materials.
This offer is rarely taken up, however, except for candidates who make the first cut. More useful perhaps is anything you have to say about your availability for interviews that might save the institution travel money. For example, if you will be attending a national conference, or are already planning a trip to the area where the institution is located, it's very helpful to say so. Then thank the committee for its consideration and sign off. The latter mistakes come easily if you've created a template letter on your computer.