Each and every domain 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 created manually if required. As an 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 reveal where the emails are managed, while using an SRV record you can use a domain for unconventional purposes, for instance a Voice-over-IP service. Being able to create and modify specific records permits you to have different services with different companies if, for example, your current host provides stable web hosting, but has troubles with the e-mail service, or you simply prefer to get a particular service from a particular company. Even though you can create and modify these records freely and with no limitations as you own a given domain address, some companies charge extra to give you this option.