Not if your resume is bad. They can be a nice differentiator if your resume is good.
Unless it's required, I wouldn't write any type of long formal cover letter. They won't read it unless your resume is strong and they already think you can do the job.
I'll put it this way: your cover letter can't save you from a bad resume. If the resume is bad, they'll skip reading whatever letter you wrote. If your resume is good, a nice, short, original (never cookie-cutter), company-specific letter can improve your odds, show you did your research and are truly interested in this specific company, separate you from similar applicants, and make a great first impression.
The best cover letters are short (1 paragraph), written from scratch, and have the same tone as the company's culture.