Recruiters read lots of cover letters. A bad cover letter is a good way Not To Get A Job.
The cover letter is an artifact from the old days of job hunting. Back then, Mr. Candidate would visit the local off-set printer, who would craft a ream of resumes that all said the same thing. Mr. Candidate would then type (or have his stenographer type) a fairly generic letter that "covered" the resume. This packet was delivered by the USPS.
Desktop publishing threw that model out the window. Today recruiters expect a cover letter and resume targeted to the position Mr. Candidate wants to fill. Cover letter styles differ not only by industry, but also by position.
ZoomInfo's recruiter Martin Burns has A Great Example of a Cover Letter that he received. While this letter can't be the model for everyone (Don't just cut and paste it!), it is a good example that many more people could follow.