Subheads convert skimmers into readers. Everyone is in a hurry these days. They don't have time to read long messages...unless they're convinced they will be well rewarded for their efforts. In most cases you message will receive only a quick once-over before it is set aside to be read until "later"--which often never comes.
Subheads to the rescue! Subheads break long text passages into bite-sized chunks. Each subhead provides a glimpse into the text that follows, "advertising" the upcoming topic. Each subhead provides an additional entry point into your message. Subheads belong in your e-mail and on your web site as well as in your ads, brochures, newsletters, and proposals.