Building an email list enables you to drive traffic at the click of a button. With a substantial number of subscribers in your email list, you can also build a community around your site or even your niche and eventually make sales after you have established relationship with your list. However, it takes time, effort and the right strategies to attract people to subscribe to your list.
Also, it is essential to use an autoresponder service to build your email list. You can check out the one I personally use: Aweber.
Other popular autoresponders are: iContact & GetResponse