the hiring process at my last job (legal field):

1 -- it gave us a short writing sample (lawyers write a lot, so this is important)
2 -- it gave us a sense of the applicant's motivations for applying for the job in ways a resume cannot. A resume tells the employer what you've done. The cover letter tells the employer what you want to do and why you want to do it.
3 -- it's a fantastic weed-out tool. Some cover letters are so incredibly generic that you realize the person has spent next to zero time looking into your company/law firm/whatever. The place I worked before had a very specific practice. We'd get people saying in their cover letters that they want to work with us because they want to do X -- something that we clearly didn't do.

Anyway, I can see how cover letters can be important.