FWIW, I spoke with 5 recruiter friends about cover letters while I was applying last year.
Their stance: ain't no one got time for that – unless it's a tiny company. Each of the 5 said they never ever read cover letters, but allowed that truly small startups, who are thus extremely selective, may put some weight into them.
Cover letters are useful for two types of candidates:
Fresh our of college (or even looking for an internship), or other non-traditional path to a technical job, where you don't have experience and need to convince me you are technical enough to interview. (this also covers people who have large gaps trying to get back into something technical)
Candidates who know they are over qualified and need to explain why they would accept the position anyway, and thus it isn't a waste of our time to interview you for a position that can't pay something reasonable.
Otherwise I read them, but they don't tell me anything. I want to see evidence you have done technical things like the type of things we need someone to do. Your resume should give me a better indication of what you can do because it is what you are doing.
The fresh out of college situation is a good point - I’d recommend a cover letter in that scenario. There isn’t enough experience to assess a candidate so makes sense to counter it with a letter.
With fresh out of college you have a degree in computer engineering just like the other 20 applicants for the one position. What you need is some reason - any - to stand out. Otherwise we will randomly interview until someone passes the interview and if you end up last on the random list you won't get an interview as odds are one of the first 5 accepts an offer. If you can stand out you can get to the top of the list, and that gives you a better chance.
When you get more experience, your experience speaks for itself. (not always a good thing - if you want to change from embedded development to front end for example you will be overlooked even though there is no reason someone cannot make that change quickly)
It varies of course – these 5 do not claim to read cover letters in either of those scenarios, and each work for important recruiting firms. They said they simply look at too many candidates a day to possibly be able to read cover letters regularly.
Depending on the person, the time spent writing a cover letter may be better put to just more applications.
One other case I've seen of a 'useful' cover letter was from someone who wanted to completely change fields. The cover letter explained while they had no education on the field and had never worked in the field, they where passionate about it and spent the past several years of free time doing it as hobby. That was enough to get them an interview, despite their 'irrelevant' CV, and the interview was good enough to get them hired
Their stance: ain't no one got time for that – unless it's a tiny company. Each of the 5 said they never ever read cover letters, but allowed that truly small startups, who are thus extremely selective, may put some weight into them.