Any active domain that is added in a hosting account includes a variety of records that allow it to work properly. In the general case, the name servers (NS records) show the location where the domain name is hosted and they are set on the side of the registrar company, but on the lower level there are certain records created through the hosting company like A (IP address of the website/server), MX (e mail server), SRV (employing a domain for non-website purposes on a particular port number such as Voice-over-IP), and many others. With access to these records and by being able to edit them, you could have your site with company A and your e-mail messages with company B, for instance. This gives you more freedom since you can pick the best provider for every single one of these services and you don't have to stick with one company. After you select a hosting company, you have to make certain that you can access all DNS records freely since some companies have an extra fee for this feature.