Not all of your site visitors are ready to talk to your sales team or see a demo of your product. Someone at the beginning of the buyer's journey might be interested in an informational piece like an ebook or a guide, whereas someone who's more familiar with your company and near the bottom of the journey might be more interested in a free trial or demo.
Every week, the folks at InVision send a roundup of their best blog content, their favorite design links from the week, and a new opportunity to win a free t-shirt. (Seriously. They give away a new design every week.) They also sometimes have fun survey questions where they crowdsource for their blog. This week's, for example, asked subscribers what they would do if the internet didn't exist.
Over time, the popularity of email marketing campaigns resulted in personal email inboxes that were overflowing with the latest and greatest offers. Email campaigns grew and people grew tired of them. Laws that allowed customers to unsubscribe breathed new life into email marketing campaigns as customers could easily pick and choose the companies they were interested in.