Every single domain which is registered and hosted has quite a lot of DNS records - some of them are set up automatically when you host a domain, others can be set up by hand if needed. As an example, the A record of a domain address is its IP address i.e. this record shows where the web site is located; the MX records show where the emails are stored, while through an SRV record you can use a domain for unconventional purposes, for instance a Voice-over-IP service. To be able to set up and modify certain records enables you to have different services with different companies if, for example, your current host provides stable web hosting, but has problems with the e-mail service, or you simply prefer to get a particular service from a specific company. Even though you must be able to create and modify these records freely and without any restrictions as you own a given domain, a lot of companies charge extra to give you this option.