If you needed any more reason to think that cover letters are mostly a waste of everyone's time, now they are worth even less. Not knocking the work put into this website.
A good cover letter
- reveals some of the personality of the applicant (I once had a software engineer describe themselves as caring and thoughtful -- that was a nice touch to complement their resume which lists technical achievements)
- gives the recruiting team a sense of passions, priorities, and style of cooperation (again, beyond the technical which is on the resume)
- lets the organisation know a little about the reason for switching jobs.
Of course, it says nothing about whether the applicant is any good at the job, but it can give a sense of whether this person is someone the team would enjoy hanging out and working with every day. I haven't verified it empirically, but I believe cover letters have been signalful for me in the past.