x509 certificate for WCF with username password authentication

[VKDD]

I used the guide lines on the link attached to create my WCF service that uses a custom username password validation. This site talks about creating a certificate using self cert utility which is used in the following section of configuration. Can someone help me understand why the certificate is needed. I don't see any mention of this in the code code username password validator class.
Guidelines link: http://www.codeproject.com/Articles/...sword-authenti

<behaviors>
        <serviceBehaviors>
            <behavior name="Behavior1">
                <serviceMetadata httpGetEnabled="true" />
                <serviceDebug includeExceptionDetailInFaults="true" />
                <serviceCredentials> 
                    <serviceCertificate findValue="MyWebSite" 
                          storeLocation="LocalMachine"
                          storeName="My" 
                          x509FindType="FindBySubjectName" />
                    <userNameAuthentication userNamePasswordValidationMode="Custom"
                     customUserNamePasswordValidatorType="CustomUsernamePasswordAuth.
                        Service.UserNamePassValidator, CustomUsernamePasswordAuth.Service" />
                </serviceCredentials>
            </behavior>             
        </serviceBehaviors>