Any active domain which is added in a hosting account has a variety of records which allow it to function properly. In the standard case, the name servers (NS records) show the location where the domain is hosted and they are 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 (email server), SRV (employing a domain address for non-website purposes on a certain 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 example. This gives you more freedom since you can choose the best provider for each of these services and you don't need to stick to one company. When you select a hosting company, you have to make sure that you can access all DNS records freely because some companies have an extra fee for this feature.