It's important to take advantage of the window of opportunity when your company or brand is at the top of your prospects' minds. You can really get a pulse of what future engagement will look like by what people do when you email then within 24 hours of their subscribing to your newsletter, signing up for an offer, and so on. Plus, it's a great opportunity for branding and setting expectations.
The Preheader references the area before the main content. It's an excellent opportunity to grab a bit of extra pre-content web real estate. It's got a degree of prominence in the inbox, so it would be a waste not to use it for promotional purposes. Try popping a couple of links in here. Some companies find that they get the most clicks on links within their preheader