Cover Letters and Thank You Letters should highlight the information in your resume that qualifies you for the job you seek.
Some Recruiters report that they do not read cover letters, but hiring managers read them because it is in the cover letter that a candidate can show who they are and their attributes. The thinking is you are applying because you believe yourself qualified for the position. As the saying, goes "Repetition is the art of memory." So do not hesitate to add a well-written cover letter that repeats a few essential facts.
Use a cover letter if the application request that you upload a cover letter or allows for the upload of a document in addition to your resume.
If you are a technical, engineering, or scientific candidate, or hold a project-based position, ask your writer to create a "skill sheet" or project list instead.
List software, tools, projects, papers, conferences etc. on a skills sheet instead.
Create a masterful intro email instead of a cover letter.
Many job applications allow for an additional document upload besides your resume; speak to your writer regarding the best use of that opportunity.