A cover letter is usually worth it when it explains something your resume cannot show cleanly on its own.
Good uses for a cover letter
It can help when you are changing fields, relocating, clarifying a strong motivation for the role, or connecting a non-obvious background to the company�s needs.
Keep it tighter than you think
Most cover letters improve when they are shorter. A few focused paragraphs usually land better than a page of recycled enthusiasm.
Bottom line
Write a cover letter when it adds context, not because you feel obliged to fill space.