Every single domain that is registered and hosted has a great deal 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 name is its IP address i.e. this record reveals where the website is situated; the MX records show where the e-mails are stored, while with an SRV record you can use a domain for unconventional purposes, such as a Voice-over-IP service. Being able to set up and modify specific records enables you to have different services with different providers if, for example, your current host provides stable web hosting, but has issues with the e-mail service, or you simply prefer to get a particular service from a particular company. Even though you will be able to set up and modify these records freely and without any limits as you own a given domain, many companies charge extra to give you this option.