Each domain that's 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 required. For instance, 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 handled, while using an SRV record you can use a domain name for unconventional purposes, such as a Voice-over-IP service. Being able to set up and modify particular records enables you to have different services with different companies if, for instance, your current host provides stable web hosting, but has issues with the email service, or you simply prefer to get a specific service from a particular company. While you will be able to set up and edit these records freely and with no limits as you own a given domain, some companies charge extra to give you this option.