Any active domain which is added in a hosting account has a variety of records that permit it to work properly. In the typical 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 set through the hosting company such as A (IP address of the website/server), MX (e mail server), SRV (using a domain address for non-website purposes on a particular port number like Voice-over-IP), and many others. Having access to these records and by being in a position to edit them, you could have your web site with company A and your e-mails with company B, for illustration. This offers you more flexibility as you can select the best provider for every one of these services and you don't have to stick to a single company. When you select a hosting company, you have to make sure that you can access all DNS records freely because some providers have an additional fee for this function.