Someone completes your form and clicks submit. Then nothing happens: no email, confirmation or indication the message was sent. You received the enquiry in your inbox, but the visitor sees a black hole and starts contacting the next company.
An automatic confirmation solves this and is among the few marketing messages you can almost certainly expect to be read: confirmations achieve open rates of 70 to 90%, far above newsletters. This article explains what belongs in one, which promises to avoid and how to check delivery.