Any active domain which is added in a hosting account has a variety of records that permit it to perform properly. In the common case, the name servers (NS records) show where the domain is hosted and they're set on the side of the registrar company, but on the lower level there are specific records created through the hosting company such as A (IP address of the website/server), MX (email server), SRV (using a domain address for non-website purposes on a particular port number such as Voice-over-IP), etc. By having access to these records and by being able to modify them, you could have your web site with company A and your e-mail messages with company B, for example. This offers you more independence as you can choose the best provider for each of these services and you don't have to stick to only one company. Whenever 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 feature.