Any active domain that's added in a hosting account has a number of records that enable it to work properly. In the standard 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 such as A (IP address of the website/server), MX (email server), SRV (using a domain name for non-website purposes on a particular port number such as Voice-over-IP), etc. With access to these records and by being able 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 freedom because you can choose the best provider for each of these services and you don't need to stick to only one company. Whenever you select a hosting company, you have to make sure that you have access to all DNS records freely as some companies have an additional fee for this function.