The answer is II: to add more detailed information about experiences and skills.
A cover letter is, traditionally, a document that accompanies your CV as its front cover and it is the perfect way to make you stand out from other applicants and get the recruiter to notice you in a more detailed and personal manner. The cover letter should be specifically written for the position you are applying for and it should be addressed specifically, if the information is available, to the person who is doing the recruiting. In it, the applicant should aim at clarifying what makes her the most suitable candidate for the position in question and how her experience and skills would be a positive asset for the company if she was given the job.
