Each domain that is registered and hosted has lots of DNS records - some of them are set up automatically when you host a domain, others can be set up manually if required. For instance, 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 e-mails are managed, while using an SRV record you can use a domain name for unconventional purposes, including a Voice-over-IP service. To be able to create and modify certain records enables you to have different services with different companies if, for instance, your current host provides stable web hosting, but has troubles with the email service, or you simply prefer to get a particular service from a specific company. Although you will be able to create and modify these records freely and with no restrictions as you own a given domain, lots of companies charge extra to give you this option.