The application letter you send with your resume provides you with your first opportunity to present yourself to a potential employer.
It is more than just a brief cover note in which you merely introduce yourself, ask the employer to accept your application, and refer to your attached documents.
Together with your resume it provides you with an opportunity to sell yourself as a highly suitable candidate for the job.
There are usually two types of application letters – one is a response to an advertised position or personal contact – the other is seeking to be considered for possible vacancies and is often called a ‘speculative’ or ‘cold canvas’ letter.