Each domain that is registered and hosted has quite a lot of DNS records - some of them are created 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 website is located; the MX records reveal where the e-mails are managed, while using an SRV record you can use a domain address for unconventional purposes, for instance a Voice-over-IP service. Being able to set up and modify particular records permits you to have different services with different companies if, for instance, your current host provides stable web hosting, but has difficulties with the e-mail service, or you simply prefer to get a certain service from a specific company. Even though you should be able to create and edit these records freely and with no limitations as you own a given domain address, many companies charge extra to give you this option.