Because SendGrid are not very clear....
Victor Acatrinei (SendGrid)
Dec 31, 00:55
The easiest way to send emails through your form is with the web API call.
When somebody fills in the form, and clicks on send, you will get the data from the form, add is as content to the email, and then send it to your email address. Of course all this needs to be done programmatically.
In case you need further assistance regarding this matter, please feel free to contact the SendGrid Support Team at any time.
We all wish you a great day!