It can be useful if used correctly but far too many people spam their lists as they think they will make money by doing so. You need to build a rapport/relationship with your subscribers as well as giving them recommendations of products/services that may help them. If they trust you they will buy on your recommendation and by treating them well you can keep selling to them over and over again.
Building relationships with potential customers and maintaining good relationships with existing buyers is going to keep you in business for a long time. Send relevant and informative content to your existing and potential customers, send them links that may benefit them in some way. You may also send them updates on the latest developments in your business or products. Sending them discount offers on new products before launching them to the general public is also a good idea. This way they stay loyal to you for a long time.
Email marketing is a VERY effective form of advertising. Many people will advise against it, but if you do it right it will allow you to build a solid list. Of course you will have better results if you send the email campaign to people who have opted in to receive your emails. In other words people who have subscribed. This ensures that people will at least receive your email. Most major chain stores use email marketing to draw old customers back by offering discounts and sales. As long as you use your creativity you will have success.