Every single domain name which is 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 set up by hand if required. For example, the A record of a domain is its IP address i.e. this record reveals where the web site is located; the MX records show where the emails are managed, while through an SRV record you can use a domain address for unconventional purposes, including a Voice-over-IP service. To be able to set up and modify certain records allows you to have different services with different companies if, for example, your current host provides stable web hosting, but has troubles with the e-mail service, or you simply prefer to get a particular service from a certain company. Though you must be able to set up and edit these records freely and without any restrictions as you own a given domain name, lots of companies charge extra to give you this option.