Each and every domain that's registered and hosted has a great deal of DNS records - some of them are set up automatically when you host a domain, others can be created manually if required. For example, the A record of a domain address is its IP address i.e. this record reveals where the site is situated; the MX records reveal where the e-mails are handled, while through an SRV record you can use a domain name for unconventional purposes, for instance a Voice-over-IP service. Being able to create and modify specific records enables you to have different services with different companies if, for example, your current host provides good web hosting, but has difficulties with the email service, or you simply prefer to get a certain service from a certain company. Though you must be able to set up and modify these records freely and with no restrictions as you own a given domain, lots of companies charge extra to give you this option.