Yes, write a cover letter. They will help you standout and express things about you that bullet points can not.
There is one scenario in which I don't write a resume. If I'm working through the process with someone I know or have an acquaintance with. Any other opportunity I am writing a cover letter to go along with a resume.
Why cover letters are important
Cover letters are a great opportunity to stand out from the pile of resumes sitting on a hiring managers desk. I recently heard some chatter that cover letters aren't relevant anymore. I would argue that they're rare. Which is exactly why you should write a cover letter for a job posting.
I used to not write cover letters. Writing a cover letter is hard. It requires inner reflection and an ability to write coherent sentences. For a non-writer that can seem daunting. I'll walk through how I write a cover letter below. I took chances in my cover letter and I was rewarded with at the very least a conversation. That's all we are looking for from a resume and cover letter, a chance for a conversation.
Cover letters are a great opportunity to show what you know and why you would be a good fit. Here are my two most recent cover letters.
You have to be very careful about pointing out issues in a website. It's like telling someone their baby is ugly. I ended up getting a call anyway. It was a short call. They were looking for someone who would jump in and start writing secure code. I was not that person. We both agreed it wasn't a great fit for them or myself.
In this example, I went much further in the interview process. I did several interviews and even made it to the sample security assessment on an application phase. This example is a little more standard. It highlights my desire to get into the appsec field and the activities I'm doing to accomplish that goal. I didn't get this role either. They were looking for someone more senior and I was looking for something closer to junior. Going deep into the process, though, was a valuable experience.
How to write a cover letter
Hopefully, those two examples are useful and provide ideas for writing a cover letter. Walking through both examples the first part of the cover letter is all the contact information. Your information and the companies information and the date.
If you have a name for the person who will review the cover letter address it to that person. I recommend not using "To whom it may concern," because there's something about the phrase that can rub people the wrong way. I like "Hiring Authority," because it empowers the person reading the letter. It provides them with a sense of importance that "to whom it may concern" doesn't.
My first paragraph focuses on the role I'm applying for and what makes me a good fit for the role. In the first example, I'm focusing more on recommendations I can make in the role. The second example, I'm trying to say that I have a strong interest in appsec, despite a weak background in development. Re-reading both first paragraphs makes me want to throw up. However, I'm keeping them (and the rest unedited) to show that a cover letter doesn't have to be an amazing thing. Try to provide a little insight into your personality. Take chances.
The middle paragraphs I'm focusing on me. What makes me a good candidate. What experience do I have. What activities I'm doing to help improve my skills in the field.
The final paragraph I focus back on the position and highlight what makes me a good fit for the role. Sort of summarizing the whole thing. Then finally sincerely your name. In example two I misspelled sincerely, which simply highlights making sure to re-read your cover letter for mistakes.
Write a cover letter to stand out
When I talk to people trying to fill a particular role, one of my questions is how many cover letters were submitted. The numbers I get from those people are very low. Cover letters give you an opportunity to standout and highlight your strengths as a candidate. Resumes are bullet points of accomplishments and responsibilities. They say very little about you as a person.
Cover letters are frustrating to write. The more you write them, the easier they become to write. I would avoid using a template. For each job you're submitting to, write a fresh cover letter. Cover letters show a willingness to go the extra mile. Which is why you may be surprised to find more calls from potential employers.