In the role of hiring manager on behalf of one of my clients, I recently posted a part time job announcement. I got many resumes from people who had clearly not even bothered to go to the website of the company to see what the place does. Worse, the cover letters I received were mostly canned ones. Many applicants did not go so far as to bother to customize their canned letters. They left the terms "your company" and "the position" as is, rather than to translate them into the actual company name and position name.
As a hiring manager, I infinitely prefer the 4-5 sentence email explaining why the applicant wants to work at OUR company over some stupid canned collection of meaningless words that some professional job coach thinks will get you an interview. It won't with me.
COME ON FOLKS!
If we are going to pay you $15 or $20 per hour for a job that a temp agency would give you $8.50 to do, is it not worth your time to put a little work into a cover letter?!
Maybe go to the website and learn about what we do?