Each domain name that is registered and hosted has a lot of DNS records - some of them are created automatically when you host a domain, others can be created by hand if needed. For instance, the A record of a domain name is its IP address i.e. this record reveals where the site is situated; the MX records show where the emails are managed, while using an SRV record you can use a domain address for unconventional purposes, including a Voice-over-IP service. Being able to create and modify specific records enables you to have different services with different providers if, for example, your present host provides stable web hosting, but has troubles with the e-mail service, or you simply prefer to get a certain service from a particular company. While you must be able to set up and modify these records freely and with no limitations as you own a given domain name, a lot of companies charge extra to give you this option.