This guy has it half right. There is some use in cover letters, although most don’t read them, but not for the reasons he suggests. It may be somewhat old-fashioned in this 140 character limit age we are in but I do think there is value for a candidate to write down in (ideally) words of several syllables exactly why the job they are applying for is suitable for them and vice-versa.
This will allow them:
-to check they are suitable for the role, how many boxes do they tick?
-to demonstrate (should someone read it) that they know how to write with correct spelling and grammar
-to practice selling themselves! This is as an opportunity to demonstrate the ways they meet the job spec and particular ways of phrasing or articulating achievements.
Hopefully this will lead to candidates engaging fully with the job they have seen advertised and be more likely to secure an interview. Equally (and I may be being a bit too ambitious here) it would sound the death toll for the generic cover letter.