One of the basic concepts of inbound marketing is offering a piece of content in exchange for contact information, usually an email address. This usually happens when someone reads a blog post, sees an offer for a free guide or similar, clicks it and is brought to a landing page which asks them for their email address in exchange for the content.
This works great but people are becoming more and more protective over their email address. I personally only ever sign up for content with my email address which I see as really valuable. Otherwise you end up with an inbox full of marketing communications that you don't want.