In this (actually pretty old) post from Seth Godin, he talks about the importance of making sure you are only trying to do one thing with any given web page. Now here's the thing, while he gives 4 different things a web page can do, the truth is that you should be striving FIRST AND FOREMOST to getting someone to "get someone to give you their email address so you can build a relationship with them."
The reason for that is simple. If you can build a relationship with your potential customers, they are much more likely to become not only paying customers, but they are also much more likely to spend more when they do it. On top of it, they are more likely to spend money with you more often.
Building a relationship with your potential customers is so important that I consider it THE single most important activity of any business (no matter what is sold or to whom).