Every single domain that's registered and hosted has a lot of DNS records - some of them are created automatically when you host a domain, others can be set up by hand if required. For instance, the A record of a domain address is its IP address i.e. this record reveals where the website is situated; the MX records reveal where the emails are stored, while through an SRV record you can use a domain address for unconventional purposes, such as a Voice-over-IP service. To be able to create and modify particular records enables you to have different services with different providers if, for instance, your current host provides stable web hosting, but has problems with the email service, or you simply prefer to get a particular service from a certain company. Though you can set up and modify these records freely and with no limits as you own a given domain address, a lot of companies charge extra to give you this option.