For any type of business to succeed, you have to get paid. So whether you do one freelance gig a month or run a full-time writing business, you need to calculate the time you've worked and bill your clients for it. That means you need to know how to write an invoice.
Creating a professional freelance invoice is vital to the success of your business. If you sent your client paper receipts and scribbled notes, they would not be as likely to pay you on time and would probably question the you run your business.