Typically when applying for jobs, a firm wants you to submit a resume and cover letter. However, most employers won't even bother reading your cover letter.
So why bother writing one?
Well, the cover letter really only comes into play if your resume is a match for what the position entails and the employer's interest in you has piqued. Once you've made it through that hurdle, the cover letter is used to further weed out unqualified candidates.