Email lists have been a foundation for essentially every business over the last several years simply because of the utility of incorporating an email list for customers to subscribe to. Having a way to directly reach customers who want to engage with your product or service holds a much higher value than otherwise broadcasting your business, hoping customers will connect with you.
Staying connected with past customers is as important or even more important than reaching new customers. Pareto's Principle of the 80-20 rule can be applied to business in the sense that 80% of your business comes from 20% of your customers. Now this is not an absolute for every business or even any business, it simply stresses the point that loyal customers make up a large percentage of a business's margin and should not be neglected. This is why so many business professionals will yell from the rooftop about how important building an email list is.