Each domain name that is registered and hosted has a lot of DNS records - some of them are set up automatically when you host a domain, others can be set up manually if needed. As an example, the A record of a domain address is its IP address i.e. this record shows where the site is located; the MX records reveal where the emails are managed, while through an SRV record you can use a domain address for unconventional purposes, like a Voice-over-IP service. To be able to create and modify certain records allows you to have different services with different providers if, for example, your present host provides dependable web hosting, but has difficulties with the email service, or you simply prefer to get a specific service from a particular company. Though you can create and edit these records freely and without any limits as you own a given domain, some companies charge extra to give you this option.
