Every single domain name which 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 created by hand if required. For instance, the A record of a domain is its IP address i.e. this record reveals where the web site is situated; the MX records reveal where the emails are stored, while using an SRV record you can use a domain address for unconventional purposes, for instance 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 instance, your current host provides stable web hosting, but has troubles with the email service, or you simply prefer to get a particular service from a certain company. Even though you will be able to create and modify these records freely and without any limits as you own a given domain, some companies charge extra to give you this option.