Every domain that is registered and hosted has quite a lot of DNS records - some of them are set up automatically when you host a domain, others can be created by hand if needed. As an illustration, the A record of a domain address is its IP address i.e. this record shows where the website is located; the MX records reveal where the e-mails 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 particular records allows you to have different services with different companies if, for illustration, 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 specific company. Though you can set up and edit these records freely and without any limits as you own a given domain, a lot of companies charge extra to give you this option.