We set up your main email account so that any mail sent to an unrecognized user at your domain is delivered to your default email address. This can be very useful because it means you can use many different email aliases without actually having to set them up explicitly. You can also give any email account an unlimited number of email aliases so that when mail is sent to alias@yourname.com you can then receive the email at, say, colleague@yourname.com (a real email address).