Each domain name that is registered and hosted has a great deal of DNS records - some of them are created automatically when you host a domain, others can be set up by hand if needed. As an example, the A record of a domain address is its IP address i.e. this record shows where the website is situated; the MX records show where the emails are managed, while with an SRV record you can use a domain address for unconventional purposes, including a Voice-over-IP service. To be able to create and modify particular records allows you to have different services with different providers if, for instance, your present host provides stable web hosting, but has troubles with the email service, or you simply prefer to get a particular service from a particular company. Although you should be able to create 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.