Each domain 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 manually if required. For instance, the A record of a domain is its IP address i.e. this record reveals where the site is situated; the MX records show where the e-mails are stored, while with an SRV record you can use a domain address for unconventional purposes, for instance a Voice-over-IP service. Being able to create and modify specific records allows you to have different services with different companies if, for example, your current host provides stable web hosting, but has difficulties with the e-mail service, or you simply prefer to get a specific service from a specific company. Though you will be able to set up and modify these records freely and with no limitations as you own a given domain address, a lot of companies charge extra to give you this option.
