Every single domain that's registered and hosted has a great deal of DNS records - some of them are created automatically when you host a domain, others can be created by hand if required. As an example, the A record of a domain name is its IP address i.e. this record shows where the website is located; the MX records reveal where the e-mails are handled, while using an SRV record you can use a domain address for unconventional purposes, for instance a Voice-over-IP service. Being able to set up and modify particular records enables you to have different services with different providers if, for example, your current host provides stable web hosting, but has problems with the e-mail service, or you simply prefer to get a particular service from a certain company. Even though you should be able to create and edit these records freely and with no limitations as you own a given domain, many companies charge extra to give you this option.