Confirmations take place after a user has submitted a form. At the most basic level, confirmations notify site users that their response has been received:
If you are simply notifying users of receipt, do not add a new confirmation message. You will need to edit the Default one. You'd only add new ones if you want to display different confirmation messages based on some conditional logics you set for the alternative confirmations, but if those conditional logics are not met then the default one will be used.
Here are the two additional options you have beyond simply notifying users of receipt:
Based on their responses (conditional logic), redirect users to another page within your website
Based on their responses (conditional logic), redirect users to a different website
These options are easy to configure and are located under Forms > the specific form > Settings > Confirmations.