Every domain name that is registered and hosted has a lot of DNS records - some of them are created automatically when you host a domain, others can be set up manually if needed. As an example, the A record of a domain is its IP address i.e. this record shows where the web site is situated; the MX records show where the e-mails are stored, while with an SRV record you can use a domain address for unconventional purposes, like a Voice-over-IP service. Being able to set up and modify certain records allows you to have different services with different providers if, for example, your current host provides stable web hosting, but has troubles with the e-mail service, or you simply prefer to get a particular service from a specific company. Even though you must be able to create and edit these records freely and with no limits as you own a given domain name, many companies charge extra to give you this option.