Any active domain that is added in a hosting account includes a number of records that permit it to work properly. In the typical 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 particular records created through the hosting company like A (IP address of the website/server), MX (e-mail server), SRV (using a domain address for non-website purposes on a certain port number like Voice-over-IP), and many others. By having access to these records and by being in a position to modify them, you may have your web site with company A and your e-mails with company B, for instance. This gives you more freedom because you can choose the best provider for every one of these services and you don't need to stick with only one company. Whenever you select a hosting company, you have to make sure that you can access all DNS records freely since some providers have an additional fee for this feature.