Any active domain that is added in a hosting account includes a number of records that enable it to perform properly. In the common 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 particular records created through the hosting company like A (IP address of the website/server), MX (e-mail server), SRV (using a domain for non-website purposes on a specific port number like Voice-over-IP), and so on. With access to these records and by being in a position to edit them, you could have your website with company A and your e-mail messages with company B, for instance. This gives you more flexibility as you can pick the best provider for each of these services and you do not have to stick to one company. After you choose 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.