They matter at smaller companies and for roles with a human reviewer; they're often ignored at large-volume employers. When they are read, the ones that work do something the resume can't: explain *why this company* and connect your specific experience to their specific problem.
**The structure that works, in about 250 words:**
*Opening (2 sentences)* — say something specific about them that proves you did more than skim the careers page. Reference their product, a recent change, something about the problem they're solving. 'I've been using [product] since [context], and the recent shift to [thing] is exactly the kind of problem I want to work on.' Delete 'I am writing to apply for the position of' — it's the sentence most likely to lose the reader.
*Middle (2 short paragraphs)* — the strongest one or two pieces of evidence that you can do *this* job, with specifics and numbers. Not a summary of your resume — the single most relevant thing, expanded with detail the resume didn't have room for. Ideally connect it to something in the job description.
*Close (2 sentences)* — what you'd bring, and a simple statement of interest. No begging, no 'I look forward to hearing from you at your earliest convenience'.
**What kills cover letters:**
- Generic openings that could be sent to any company. Reviewers spot these instantly and stop reading.
- Repeating the resume in prose. If it adds nothing, it costs you.
- Adjectives without evidence: passionate, dedicated, hard-working, team player. Every applicant claims these; none of them are checkable.
- Length. Over one page, or over about 300 words, and it's being skimmed at best.
- Making it about what you want from them, rather than what you'd contribute.
**The test**: could this letter be sent to a different company by changing the name? If yes, it's doing nothing. The specificity *is* the value — it's evidence of genuine interest, which is what the letter exists to demonstrate.
**When to skip it**: if it's optional, the company is very large, and you're applying at volume, your time is better spent tailoring the resume and finding a referral. When to invest in it: a company you genuinely want, a small team, a role where communication matters, or when your resume needs explaining (career change, gap, non-obvious fit). In those cases it's the highest-leverage document you have.