Your email list is a major component for marketing to your fanbase. It's widely overlooked since most artists believe that their Facebook friends and Twitter followers are enough, but you email list allows you to reach out and personally connect with the fans and control you message while you're doing it.
A well thought out email blast allows you to do the following:
1) Engage your fan on a more 1 to 1 basis
2) Design the communication without the constraints of a social network
3) Add a call-to-action
This makes it easier to inform, market and sell to your fan in a manner that the true fan (superfan, uberfan, Tribal member - whatever you want to call them) enjoys, if you do it well.
Here's a short video about the best way to harvest those valuable email addresses.