Email is everywhere; on desktop, mobile, and now even on watches.
As a marketing tool it's a relatively inexpensive way to keep previous customers informed of new developments, and maintain mindshare for potential customers. Coupons, discounts, and limited-time offers can drive traffic to your physical and online store. Plus, with sign-up verification, it's a communication channel customers are literally asking you to use to stay in touch.
Due to its incredible flexibility, email is a workhorse in the marketing stable.
That's why I've enjoyed working on email marketing campaigns for the past several years.