Ask HN: Writing cover letters for tech jobs

I really hate writing cover letters as I never know what to write or if anyone is even going to read them. I see a lot of sites offering advice on how to write generic cover letters, but most all of them don’t seem appropriate (at least to me) for tech jobs - more for formal sales, business jobs. I'm interested to know what HN’ers with experience on either or both sides have to say by way of advice - What do you usually write/expect, is it even really a requirement?. Do you attach a separate document or just write an informal email. What tone do you take - formal, familiar. Do you summarize your skills experience or just include a link to Github etc.