Every single domain name that's registered and hosted has a lot of DNS records - some of them are set up automatically when you host a domain, others can be created manually if needed. For instance, the A record of a domain is its IP address i.e. this record shows where the website is located; the MX records show where the emails are managed, while with an SRV record you can use a domain address for unconventional purposes, such as a Voice-over-IP service. Being able to create and modify certain records permits you to have different services with different companies if, for example, your present host provides dependable web hosting, but has troubles with the email service, or you simply prefer to get a certain service from a certain company. Though you can create and edit these records freely and with no restrictions as you own a given domain name, a lot of companies charge extra to give you this option.