You can attach files—such as a detailed time report, design drafts, or additional project information—to an invoice before sending it to your client. You can also attach an Expense Report, which will contain receipt images entered with your expenses.
The best thing about using an invoice system is that you can track your time against the amount you're getting paid for each task. In order to run a successful business, you want your hourly rate to be profitable so you're not losing time and money. Another benefit of using an invoicing system is that you'll never save over an invoice and lose the work you've calculated for another client.