When a form is completed on your website you might want to send the person completing the form an email with a message. For example you might want to simply say thank you and to confirm that the form has been submitted and you will get back to them soon. Or if you have a registration form you might want to send them details about their registration and what to do next.
There may be many scenarios where you would like to send an email after someone completes one of the forms on your website.
Here is a short video explaining how to set that up.