The purpose is exactly as the title says: Sales Invoice Totals by Item. The same thing is true of Sales Invoice Totals by Customer and Sales Invoice Totals by Custom Field. Program behavior is exactly as designed and described in this Guide: https://guides.manager.io/12310.
These reports were created at the request of many users. You might not like them, but others do. Suppose you wanted to rank buying history of customers. The Sales Invoice Totals by Customer report gives you that. If you want complete customer history, that’s available in the Customers tab.
Or suppose you wanted to compare sales of an item in two different seasons to assess the effectiveness of an advertising campaign. Sales Invoice Totals by Item would tell you that information. The fact that a customer later returned an item has nothing to do with the effectiveness of the advertising campaign. And you can find such information elsewhere in the program.
The fact that a particular report does not give the exact information you might want does not mean it is useless to others. If you want details of all sales, drill down on the sales income account, where you will have both cash and credit sales.
