Take everything you have ever heard, read, or seen about cover letters and throw it out the window! That's right, 99% of the information written about cover letters is useless. Contrary to some of the more fashionable books on job search, no one ever got a job because of a great (or "perfect") cover letter. Cover letters are extremely limited in value, even when properly used. But they do have their place, when used properly.
Why are cover letters limited in value? Three reasons. First, most people assume that their cover letter is actually read before the resume. Wrong. Just ask those who spend any portion of the work day reviewing resumes—they bypass the cover letter and go directly to the resume. They only look at the cover letter if they are still interested after their initial resume review. In my review of more than 100,000 resumes over the past twenty plus years, I have probably read less than 10,000 cover letters. It is actually rather amusing to watch a Hiring Manager reading a newly arrived resume. The cover letter is ignored and the resume is scanned first, then read. And you know there is interest if they finally make their way back to the cover letter.
Second, most people assume that the cover letter should be about you. Wrong again. It should be about the employer, your prospect, your target. Your resume will tell them everything they need to know about you (if it is well written). If you are interested enough in the company to make an initial contact, take the time to fully reflect your understanding of the company and how you may be able to meet their needs in your cover letter.
Third, and most important, many college students end up using their cover letter and resume as part of a mass mailing/emailing/posting to convince themselves that they are actually doing something in their job search. "But I submitted over two hundred resumes!" In reality, all they are doing is generating rejection letters. Mass mailing/emailing/posting of your cover letter and resume has extremely low odds for success in today's job market.
Please understand that at the entry level a resume and cover letter on their own do little good. Most larger companies have established college recruiting programs which serve as the focal point for their entry level hiring. Therefore, unsolicited entry level resumes are often ignored. Many small and medium-sized companies do not have the internal resources necessary to train entry level hires, so the entry level resumes are simply filed in the virtual trash can (i.e. internal resume database/ATS) . The best you can hope for in a blind mailing/emailing/posting campaign is that you will be filed away and perhaps miraculously resurrected at some future date. Highly unlikely.