Every domain name which is registered and hosted has a lot of DNS records - some of them are set up automatically when you host a domain, others can be set up by hand if necessary. For example, the A record of a domain name is its IP address i.e. this record shows where the web site is located; the MX records show where the e-mails are stored, while with an SRV record you can use a domain for unconventional purposes, like a Voice-over-IP service. To be able to create and modify specific records allows you to have different services with different providers if, for instance, your current host provides stable web hosting, but has problems with the email service, or you simply prefer to get a certain service from a particular company. Although you can create and edit these records freely and without any limits as you own a given domain address, a lot of companies charge extra to give you this option.