At least in tech circles, I haven't heard of anyone requiring or preferring a cover letter.
There’s no job I’m keen enough on to go through all that trouble.
Also because the few times I did actually write something like that it just disappeared into a black hole.
If I am reaching out to you Daniel because I think I can add value to your company/team then it makes sense to write a custom introduction email. This can apply to an individual or reaching out to some startup via a contact email. For positions where one is applying through a form on a recruiting website I think the magic is lost at that point. Companies should not be asking for one in the recruiting pipeline. Sure it could help the applicant or the interviewee but in the general case it will just create noise.