Any active domain which is added in a hosting account has a number of records that permit it to work properly. In the general case, the name servers (NS records) show the location 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 (employing an Internet domain for non-website purposes on a particular port number like Voice-over-IP), and many others. By having access to these records and by having the capacity to edit them, you may have your web site with company A and your e-mails with company B, for instance. This offers you more flexibility as you can select the best provider for each of these services and you don't need to stick to only one company. As soon as you pick a hosting company, you have to make sure that you have access to all DNS records freely because some companies have an extra fee for this feature.