Your cover letter is just as important as your resume. You have written your resume as a document that represents you, and the recruiting manager has written a position description that reflects the organization’s needs. A cover letter ties them together. It’s more specific than a resume and highlights projects or efforts that align with the requirements of the position or program.
You should plan to submit a cover letter along with your resume (or CV) for every position or program you apply for. The only exception is if the internship/job posting specifically notes not to include one.