Every single domain that's registered and hosted has quite a lot of DNS records - some of them are created automatically when you host a domain, others can be set up by hand if needed. As an example, the A record of a domain is its IP address i.e. this record reveals where the site is located; the MX records reveal where the e-mails are managed, while using an SRV record you can use a domain for unconventional purposes, like a Voice-over-IP service. To be able to set up and modify specific records permits you to have different services with different companies if, for instance, your present host provides good web hosting, but has problems with the email service, or you simply prefer to get a particular service from a particular company. While you should be able to set up and modify these records freely and with no limitations as you own a given domain name, some companies charge extra to give you this option.
