Any active domain that's added in a hosting account includes a variety of records that enable it to function properly. In the general case, the name servers (NS records) show where the Internet domain is hosted and they're set on the side of the registrar company, but on the lower level there are specific records set through the hosting company such as A (IP address of the website/server), MX (e mail server), SRV (employing a domain address for non-website purposes on a specific port number like Voice-over-IP), and many others. With access to these records and by having the ability to edit them, you can have your site with company A and your emails with company B, for example. This offers you more freedom since you can pick the best provider for each of these services and you don't need to stick with only one company. After you select a hosting company, you have to make sure that you have access to all DNS records freely as some companies have an additional fee for this function.