Each domain which 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 situated; the MX records show where the emails are managed, while using an SRV record you can use a domain name for unconventional purposes, for instance a Voice-over-IP service. To be able to set up and modify particular records permits you to have different services with different companies if, for illustration, your current host provides stable web hosting, but has problems with the email service, or you simply prefer to get a particular service from a certain company. Although you will be able to create and modify these records freely and without any restrictions as you own a given domain address, a lot of companies charge extra to give you this option.