"You never have a second chance to make a good first impression", is equally valid for Cover Letter. To introduce your sample resume to employer, cover letter plays important role. Your cover letter tells that who you are, what you are, why an employer should spend their valuable time for you.
Cover letter is an opportunity by which you can:

Cover letter not only conveys your interest and qualifications, but also makes an employer to observe your written skills, grammar, spellings etc. To put you ahead of the competition, a well-written cover letter is the main key. 

In addition to your administrative skills, experience and qualification, a properly written cover letter shows your interest, desire to work for the company/firm and in what ways you can do beneficial work for firm. These are some examples related to Administrative assistant position.