Any active domain which is added in a hosting account includes a variety of records that enable it to work properly. In the common case, the name servers (NS records) show where the domain 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 such as A (IP address of the website/server), MX (email server), SRV (employing a domain name for non-website purposes on a specific port number like Voice-over-IP), etc. With access to these records and by being in a position to modify them, you may have your site with company A and your e-mail messages with company B, for instance. This offers you more freedom because you can select the best provider for every single one of these services and you don't need to stick to a single company. As soon as you choose a hosting company, you have to make sure that you can access all DNS records freely because some providers have an additional fee for this feature.