Every domain that's registered and hosted has lots of DNS records - some of them are set up automatically when you host a domain, others can be set up manually if needed. For instance, the A record of a domain address is its IP address i.e. this record shows where the website is located; the MX records reveal where the emails are managed, while through an SRV record you can use a domain for unconventional purposes, such as 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 present host provides good web hosting, but has difficulties with the email service, or you simply prefer to get a particular service from a specific company. Though you should be able to set up and modify these records freely and with no restrictions as you own a given domain name, lots of companies charge extra to give you this option.