The Form Builder lets you create custom email template for your visitors that will help increase your engagement with them. Writing custom emails are important if you want to connect with your visitors on a personal level, default templates gives you a limitation with a fixed mail body and mail footer.
This option is available when you choose your submit action as User Mail or Admin Mail. Both these actions have the option to customize the email template received by the submitter.
Just enable Custom Email Template option and you will then be provided with a space to write any custom message or any code however you want the email template to look like. You can even use Smart Tags to make things easier in customization.