SMTP Mail is a handy thing to have on any web site, whether for orders or just user feedback. .NET provides you with the System.Web.Mail.MailMessage class which has everything you need to send mail.
You will notice that the System.Web.Mail.SmtpMail class is assigned to a variable before it is used. This is not necessary if you are going to be using the SMTP service on your web server, you can directly call the System.Web.Mail.SmtpMail.Send method.
However the ability to nominate an SMTP server is a nice feature, as it aids administration, in that you have only one SMTP server to worry about configuring and securing.