The short answer
Send one when the employer requires it. When it is optional, write it if you can add useful, specific context that the resume does not communicate on its own.
Read the instructions before deciding
A required attachment or explicit request is part of the application. Check whether the employer asks for a conventional letter, a short response, a writing sample, or answers to named questions. Those are not interchangeable documents.
When a portal marks the letter optional, that usually tells you the form can accept an application without it. It does not prove how every recruiter will evaluate the choice. There is no reliable universal rule that all letters are read or that none are read.
When a letter can add something
A letter can explain a career change, connect unusual experience to the role, or show a specific reason for choosing that work. It can also help when writing or stakeholder communication is central to the position and the document demonstrates those skills.
- You have a relevant project that needs a little context to make sense.
- You are moving between fields and can explain the transferable work.
- You can explain a concrete connection to the employer's product or responsibilities.
- The employer explicitly invites an explanation that does not fit the resume.
When the draft needs more work
A letter that only changes the employer name while repeating generic adjectives adds little information. Before investing more time, identify the one point you want the hiring team to remember. If you cannot find a relevant point, review the fit of the application rather than padding the letter.
Do not let an optional letter distract you from required questions, accurate dates, a readable resume, and the correct documents. A polished greeting does not repair an incomplete application.
Check the final application
If you include a letter, use the requested format and inspect the exact file or text that will be submitted. Confirm the right employer, role, and version. If the site has no cover-letter field, do not replace a requested resume with a letter or force the text into an unrelated question.
Keep a copy with the resume version and job description. That makes later interview preparation easier and helps you avoid sending contradictory details across several applications.
Sources and further reading
Provider and employer requirements can change. Follow the instructions for the particular application or integration.