Many job seekers focus most, if not all, of their energy on developing the resume while giving little thought to the cover letter; others are unsure what a cover letter does or why it is even needed. As the document that personally introduces you (and your resume), it can be a powerful and essential tool in a job search.
Under most circumstances, a cover letter is used as an introduction and should accompany your resume, whether e-mailed, posted on a job site, mailed, or hand delivered to an employer.
There are a few exceptions to this rule: being a personal introduction that accompanies your resume, a cover letter is not needed if you are hand carrying a resume to a prearranged interview since you will be present to introduce yourself. If an interview is not prearranged when hand delivering a resume, then a cover letter is appropriate since you would not want to be so presumptuous as to assume you will have an interview — even if that turns out to be the case.
Also, if you are delivering the resume to a recruiter or during a networking event such as a career fair, a cover letter is not always needed as these are often cases where the opportunity to introduce oneself in person supersedes the need for a written introduction. However, many candidates at career fairs are handing out resumes; to be sure your documents stand out, you may want to have a broad cover letter on hand and include it with your resume when not meeting directly with hiring managers.