Each domain name that is registered and hosted has a great deal of DNS records - some of them are set up automatically when you host a domain, others can be created by hand if necessary. For example, the A record of a domain name is its IP address i.e. this record shows where the site is located; the MX records show where the e-mails are managed, while through 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 certain records allows you to have different services with different companies if, for instance, your current host provides good web hosting, but has issues with the email service, or you simply prefer to get a particular service from a certain company. Though you should be able to set up and modify these records freely and without any limitations as you own a given domain, a lot of companies charge extra to give you this option.