I disagree strongly with Piotr Hajlasz's answer. I've been on a zillion searches for my US-based university.
At a liberal arts college, the cover letter is extremely important
The cover letter is where you get to showcase your passion for being a professor (teaching, research, service, undergrad research, etc.) and show that you're a good fit for the university. It's not a bad idea to quote from the mission statement, and to explain why you want to be at a place where you'll have the opportunity to shape students' lives, where developing deep and meaningful relationships with students and colleagues is encouraged, and also to communicate briefly about your teaching philosophy, experience, research experience, and research direction. Normally you start with why you want to be at this particular university, then do one paragraph about each of teaching and research, plus some mention of service.
It seems to me that even for a postdoc at an R1 university in the US, having a good cover letter helps, because passion, communication skills, collegiality, care about quality teaching, and expressing that you actually like all aspects of the job, can help put the search committee in a good mental frame of mind to read your file. It's also a chance to distinguish yourself from the hundreds of other applicants. Postdocs do need to teach and be good departmental citizens (maybe you want to help organize the seminar? Great! Put that in the cover letter), and places don't want to hire someone who is going to cause trouble.
I recommend writing a template cover letter for each different type of job you plan to apply for (e.g., postdoc, VAP, tenure track) and then tailoring the template to each place. Doing that is also a good way to learn a bit more about special programs these universities have that you might connect into (do they have a seminar in your area? Do they have a summer research program? Some program to incentivize interdisciplinary research?), and people you might work with. Maybe you find that by reading the website more carefully in order to write your cover letter, you don't actually want the job after all. Or, maybe reading that material makes you realize you're a perfect fit for this university, and you can express that in the cover letter.