The primary reason for using statements in QuickBooks is to remind delinquent customers that they owe you money and to summarize the amount they owe. However, some businesses use statements instead of invoices.
Using QuickBooks Statements as reminders
If you bill your customers via invoices, then a statement is a great way to summarize what a certain customer who is delinquent in paying you owes. Statements look similar to invoices, but they list each transaction (invoices sent, credits given, payments received) for which there is an outstanding balance, the date of the transaction and then the total amount due. Because statements simply summarize information already in QuickBooks, they do not require any data entry. Because they are very easy to create for one or all delinquent customers, I often recommend to my customers that they create statements for all delinquent customers on a regular basis, and then quickly review them to decide which ones to send out.
See Creating Statements in QuickBooks for All Open Transactions for a step-by-step QuickBooks tutorial on creating statements in QuickBooks.
Using Statements instead of invoices in QuickBooks
Some businesses that bill periodically (weekly, monthly, etc.) choose to send statements to their customers instead of invoices. Instead of writing invoices, these businesses record charges to their customers in the customer register. Then on a regular basis they simply create statements which summarize the entries which have been made in the customer register. While this makes sense for some customers, using billing statements instead of invoices has significant limitations.
See Using the QuickBooks Customer Register Instead of Invoices for a step-by-step QuickBooks tutorial on this topic.