Every domain name which is registered and hosted has a lot of DNS records - some of them are set up automatically when you host a domain, others can be set up manually if required. For example, the A record of a domain name is its IP address i.e. this record reveals where the website is located; the MX records show where the e-mails are stored, while using an SRV record you can use a domain address for unconventional purposes, like 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 current 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. While you must be able to create and edit these records freely and with no limits as you own a given domain, a lot of companies charge extra to give you this option.