Each domain that's registered and hosted has a great deal of DNS records - some of them are created automatically when you host a domain, others can be set up manually if needed. For instance, 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 managed, while through an SRV record you can use a domain for unconventional purposes, including a Voice-over-IP service. To be able to set up and modify certain records allows you to have different services with different providers if, for example, your current host provides dependable web hosting, but has problems with the e-mail service, or you simply prefer to get a specific service from a certain company. Though you can create and modify these records freely and with no restrictions as you own a given domain name, lots of companies charge extra to give you this option.