A customer that feels valued and appreciated will continue to do business with you. Assuming just because they did business with you once that they will continue to do so is a mistake that can cost you a lucrative client. Regularly communicating with your customers via intelligent email marketing campaigns to share knowledge and advice shows that you care about their success and strengthens your connection to them.
One of the best ways to stay visible to your clients is to provide useful information at the right time. Collecting the email address is just the first step. What you do with those email addresses can be the difference between building a loyal customer base or chasing them to your competitors.