Every domain name that's 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 instance, the A record of a domain name is its IP address i.e. this record reveals where the site is situated; the MX records reveal where the e-mails are stored, while through an SRV record you can use a domain address for unconventional purposes, such as a Voice-over-IP service. To be able to create and modify particular records permits you to have different services with different providers if, for example, your current host provides dependable web hosting, but has troubles with the e-mail service, or you simply prefer to get a particular service from a specific company. Although you must be able to set up and modify these records freely and without any limitations as you own a given domain, a lot of companies charge extra to give you this option.