Any active domain which is added in a hosting account has a variety of records that permit it to function properly. In the typical 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 certain records created through the hosting company such as A (IP address of the website/server), MX (email server), SRV (using an Internet domain for non-website purposes on a specific port number such as Voice-over-IP), etc. By having access to these records and by being in a position to modify them, you could have your web site with company A and your e-mail messages with company B, for example. This gives you more flexibility since you can select the best provider for every one of these services and you don't have to stick with one company. Whenever you choose a hosting company, you have to make sure that you can access all DNS records freely because some companies have an extra fee for this function.