Each domain which 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 necessary. For example, the A record of a domain is its IP address i.e. this record reveals where the site is situated; the MX records reveal where the emails are handled, while using an SRV record you can use a domain for unconventional purposes, for instance a Voice-over-IP service. Being able to set up and modify particular records allows you to have different services with different providers if, for instance, your present host provides good web hosting, but has issues with the e-mail service, or you simply prefer to get a certain service from a certain company. Though you should be able to set up and modify these records freely and without any limitations as you own a given domain name, lots of companies charge extra to give you this option.