Each 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 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 situated; the MX records show where the e-mails are handled, while with an SRV record you can use a domain name for unconventional purposes, such as a Voice-over-IP service. Being able to set up and modify particular records enables you to have different services with different companies if, for instance, your current host provides dependable web hosting, but has issues with the e-mail service, or you simply prefer to get a certain service from a particular company. While you should be able to set up and modify these records freely and with no limits as you own a given domain, lots of companies charge extra to give you this option.
