Every domain name which is registered and hosted has lots of DNS records - some of them are created automatically when you host a domain, others can be set up by hand if needed. For instance, the A record of a domain is its IP address i.e. this record shows where the site is situated; the MX records show where the e-mails are stored, while using an SRV record you can use a domain name for unconventional purposes, like a Voice-over-IP service. Being able to create and modify particular records enables you to have different services with different providers if, for example, your current host provides stable web hosting, but has difficulties with the email service, or you simply prefer to get a specific service from a certain company. Although you should be able to set up and modify these records freely and without any limitations as you own a given domain name, a lot of companies charge extra to give you this option.