Any active domain name that is added in a hosting account includes a number of records which allow it to function properly. In the standard case, the name servers (NS records) show where the domain name 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 an Internet domain for non-website purposes on a specific port number such as Voice-over-IP), and many others. Having access to these records and by being able to edit them, you can have your web site with company A and your e-mails with company B, for instance. This offers you more overall flexibility as you can select the best provider for each of these services and you don't need to stick with a single company. When you select a hosting company, you have to make sure that you have access to all DNS records freely because some providers have an additional fee for this function.