Any active domain name which is added in a hosting account has a number of records that permit it to work properly. In the common case, the name servers (NS records) show the location 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 (email server), SRV (using a domain name for non-website purposes on a specific port number like Voice-over-IP), and many others. By having access to these records and by having the capacity to modify them, you could have your website with company A and your emails with company B, for illustration. This offers you more freedom since you can pick the best provider for every one of these services and you don't need to stick to a single 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.