A common trend I've noticed is people completely disregarding a cover letter as part of their application process. I know it's tempting when a job posting lists the cover letter as optional to ignore this step, especially when the whole job hunting is so involved and you're ready to just submit the application already. But it's imperative that you follow through with this.
You wouldn't think twice about submitting your resume when applying for a job, so why the trepidation with a cover letter? Maybe some see it as simply a repetitive copy of their resumes, hashing out the same skills and abilities all over again - I mean seriously, who has time for that? But here's the rub, that's really not a cover letter's purpose. Your resume is built to outline your basic experience and accomplishments, ideally to catch a hiring manager's eye to delve a little deeper. A resume does not wield its full power without a cover letter to add context and depth to it.
Here, in this quick but powerful letter, you can extrapolate your concrete skills from your work history. You let your potential future employer know the specific job posting you are applying to as well as your qualifications directly related to the position. How will these awesome abilities translate into you being a good fit? You want to make the hiring manager see just what you will be able to bring to the table. You'll also establish your voice, which can help the employer asses your personality and feel out if you would be a great fit for their company.
A cover letter is also extraordinarily useful if you have anomalies on your resume. Are you applying for a job in a completely different state than where you are currently living? Let 'em know you are looking to relocate. Is there a significant gap in employment? Explain why. The cover letter is truly your best chance at making your complete case for having a potential employer call you for an interview.