Any active domain which is added in a hosting account features a variety of records which allow it to perform properly. In the typical 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 set 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 certain port number such as Voice-over-IP), and so on. By having access to these records and by being in a position to modify them, you could have your web site with company A and your e-mails with company B, for example. This offers you more independence since you can select the best provider for each of these services and you don't have to stick to a single company. After you pick a hosting company, you have to make certain that you have access to all DNS records freely because some providers have an additional fee for this feature.