Each domain which is registered and hosted has quite a lot of DNS records - some of them are created automatically when you host a domain, others can be created manually if required. For instance, the A record of a domain is its IP address i.e. this record shows where the site is located; the MX records reveal where the e-mails are stored, while with an SRV record you can use a domain name for unconventional purposes, for instance a Voice-over-IP service. To be able to set up and modify particular records enables you to have different services with different providers if, for example, your present host provides good web hosting, but has issues with the e-mail service, or you simply prefer to get a specific service from a specific company. Even though you can create and edit these records freely and with no limitations as you own a given domain, lots of companies charge extra to give you this option.