Many businesses receive the majority of their web traffic from Google. Google isn't a controllable traffic source, though, because its ranking algorithms change regularly. Your business may receive plenty of web traffic from Google now, but an algorithm change could cause your traffic -- and your earnings -- to suddenly decrease. Building an email list can help your company weather difficult times because it gives you a source of traffic that you can control. Have you released a new product? Do you have a sale coming up? Broadcast it to your mailing list. If your list is large enough, you'll earn money from every mass message that you send.