Headlines are, without a doubt, one of the most important elements in copywriting. As has been said countless times before, if you don't get your readers' attention with the headline, the chance of them reading your copy is virtually none. But headlines (and sub-headlines) play a vital part in the copywriting process for other reasons, too.
Whether we, as copywriters, like it or not, the fact is most people will not read all the copy word for word. Headlines can help fill in the blanks in several ways so "skimmers" still get the gist of the information included in the copy.