Each domain name 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 set up by hand if needed. For example, the A record of a domain is its IP address i.e. this record shows where the web site is situated; the MX records show where the emails are handled, while using 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 allows you to have different services with different providers if, for example, your current host provides stable web hosting, but has troubles with the email service, or you simply prefer to get a specific service from a certain company. Although you can create and modify these records freely and with no restrictions as you own a given domain name, many companies charge extra to give you this option.