Newsletters are an effective tool for businesses to collect email addresses, convert visitors into buyers, and keep in contact with existing customers. They prevent customers from forgetting about the business or being won over by competitors. The frequency of newsletters depends on factors like what is being sold and how often subscribers want updates, though a good rule of thumb is once per week or twice per month. Newsletters should provide industry news, tips, and announcements about new products or services while consistently maintaining a familiar format and including unsubscribe links.