Any active domain that is added in a hosting account features a variety of records that allow it to work properly. In the common case, the name servers (NS records) show where the Internet domain 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 like A (IP address of the website/server), MX (email server), SRV (using a domain for non-website purposes on a specific port number like Voice-over-IP), and many others. With access to these records and by being in a position to edit them, you could have your site with company A and your e-mail messages with company B, for example. This gives you more overall flexibility as you can pick the best provider for every single one of these services and you don't need to stick with only one company. When you select a hosting company, you have to make sure that you can access all DNS records freely since some companies have an additional fee for this function.