Every single 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 created by hand if needed. As an example, the A record of a domain is its IP address i.e. this record shows where the web site is located; the MX records show where the emails are managed, while with an SRV record you can use a domain for unconventional purposes, for instance a Voice-over-IP service. To be able to create and modify specific records allows you to have different services with different companies if, for instance, your current host provides dependable web hosting, but has problems with the email service, or you simply prefer to get a certain service from a certain company. Even though you must be able to create and modify these records freely and with no restrictions as you own a given domain address, lots of companies charge extra to give you this option.