A letter you write to accept an interview or to ask for an interview can help a recruitment agency assess your suitability for a role and could be a key factor in their decision making process. These letters provide background information on yourself in your own voice, and can lead to you being interviewied for your chosen position.
A letter sent speculatively requesting an interview provides us with a valuable insight into your experience and your personality, so you'll want to choose your words carefully in order to make a solid first impression. In such a letter, you'll want to mention your work experience, hands-on skills, education and more in hopes that you'll secure an interview with the company.
use a covering letter to set yourself apart
Your letter accepting or requesting an interview can also be used as a platform to set yourself apart from other candidates that may want to interview for the same positions. In a letter, you can inject a little bit of your own personality to show employers why you would be agreeable to work with in the office, though you should always remain courteous and polite.
Since you'll likely send along a current version of your CV along with any letter of interest for interviews, it will be important you don't spend the entire letter restating what's on your CV. Use your letter to expand on certain experiences in your work history or to further mention your interests or work that pertains to the job you would like to be interviewed for.