Each domain that is registered and hosted has lots of DNS records - some of them are set up automatically when you host a domain, others can be set up by hand if needed. For example, the A record of a domain name is its IP address i.e. this record reveals where the website is located; the MX records show where the emails are handled, while with an SRV record you can use a domain name for unconventional purposes, like a Voice-over-IP service. Being able to set up and modify specific records permits you to have different services with different providers if, for illustration, your present host provides good web hosting, but has difficulties with the email service, or you simply prefer to get a certain service from a particular company. While you will be able to set up and modify these records freely and with no restrictions as you own a given domain name, a lot of companies charge extra to give you this option.