A newsletter is an effective tool for businesses to keep existing customers informed of new products and services, prevent customers from forgetting about the business or being won over by competitors, and collect email addresses from interested visitors to convert them into buyers. Newsletters should be sent once per week or twice per month, include relevant industry news, tips, or information about the business's products/services, and maintain a consistent format with clear unsubscribe links to build trust and familiarity with subscribers over time.