Often the cover letter is the first contact you have with a potential employer. A cover letter should complement not duplicate your resume.
One of the most important characteristics of a cover letter is showing your ability to present in written form. It also provides the opportunity to apply your life experiences to the job and allows for a personal touch that can help you stand out.
[I]f there is any way to find out the name of the hiring manager or recruiter (through people you may know at the company, the job posting, etc.) you can certainly address it to them.
Please understand that I truly believe you need a good resume and cover letter to help in the process, but I recommend using the majority of your time networking and researching the position(s) you are interested in exploring.
A cover letter provides you with the opportunity to distinguish yourself for a role with your own words, more so than a resume may be able to. Within a cover letter you have the opportunity to help frame the story that relates to your skills, experiences, accomplishments and career objectives.
Some larger companies require applicants to submit a resume and cover letter in preferred formats, as they are uploaded into an applicant database.