Each domain name that is registered and hosted has lots of DNS records - some of them are set up automatically when you host a domain, others can be set up manually if required. As an illustration, 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 stored, while with an SRV record you can use a domain address for unconventional purposes, including a Voice-over-IP service. To be able to set up and modify specific records allows you to have different services with different companies if, for example, your present host provides stable web hosting, but has problems with the email service, or you simply prefer to get a specific service from a certain company. Though you will be able to create and modify these records freely and without any restrictions as you own a given domain address, lots of companies charge extra to give you this option.