The official version goes that a cover letter shows your motivation and your hunger to get the job. If you do not take the time to write a cover letter, you don’t really want the job no?? A more down to earth version goes that it helps the junior HR staff to preselect the short list of potential candidates. After all, if you get 100 cv’s, you need some help in drilling down… Would be ok if you believe it’s HR who does the final decision, but in a phase of just filtering the bad ones out, all you need is a CV.
But in my opinion, it’s all BS: If a recruiter only takes time to write a half pager with some generic stuff (you know the drill: open minded, team-player, 5-10 years experience in a BtB market, project experience,…), how do you expect somebody to already want the job with all their hart?? People who say this are only interested in either the company name (“it’s the coolest brand ever”) or in the job-title (“look at my new business card”), none of them are good reasons to hire somebody.
And yes, I know, you would love to see me telling how great your company is (it probably is, otherwise the applicant would not even go through the trouble of reaching out, asking to open up the conversation), but that should not convince me to hire me either don’t you think?
Wanting a job really shows trough interaction with people, by talking to people working there, by interacting with the company twitter, by walking to the interview space in their offices.