Any active domain name which is added in a hosting account includes a variety of records that allow it to work properly. In the common 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 certain records created through the hosting company such as A (IP address of the website/server), MX (e-mail server), SRV (employing a domain for non-website purposes on a certain port number such as Voice-over-IP), and many others. Having access to these records and by having the capacity to modify them, you can have your web site with company A and your emails with company B, for illustration. This offers you more overall flexibility since you can select the best provider for each of these services and you don't need to stick to one company. After you pick a hosting company, you have to make sure that you have access to all DNS records freely because some providers have an extra fee for this function.