When active users have opted-in to receive e-mail communication, e-mail marketing is an extremely effective channel for customer retention.

It isn't as effective for user growth via purchased e-mail lists because recipients are not users yet and may not have opted-in - they haven't developed a relationship with your business, so when they receive e-mails, they aren't recognizing them as coming from a trusted source.

Every business should be utilizing e-mail marketing and marketing automation to communicate with their users / customers and improve retention.