why do I need in web.config?

Hi; 
What I am understanding so far: the mailSettings in web.config file will be used when there are any "Register", "Forget Password" controls in my website, so the mailSettings will be used for sending emails.
such as:
<system.net>
        <mailSettings>
            <smtp deliveryMethod="Network" from="support@mydomain.com">
                <network host="mail.hostingsite" port="25" userName="support@mydomain.com" password="mypassword" defaultCredentials="false"/>
            </smtp>
        </mailSettings>
    </system.net>
 
What if I put all the necessary inf ...