A job posting is an expression of pain. They have specific gaps they're looking to fill. Your job is to communicate you understand their pain and can alleviate their suffering.
The best cover letters tell a short story. Connect your experience to their needs. Don't repeat your resume — add context. Why are you interested in this company? What do you bring that others don't?
Keep it concise. Three short paragraphs max. Hiring managers are busy. Respect their time.
Some roles don't require a cover letter. When in doubt, include one. It's another chance to stand out.