Any active domain which is added in a hosting account has a number of records that allow it to function properly. In the general 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 particular records created through the hosting company like A (IP address of the website/server), MX (e mail server), SRV (using an Internet domain for non-website purposes on a certain port number like Voice-over-IP), and so on. With access to these records and by having the ability to edit them, you could have your site with company A and your e-mail messages with company B, for instance. This gives you more freedom since you can pick the best provider for every one of these services and you don't have to stick with only one company. When you select a hosting company, you have to make certain that you can access all DNS records freely as some companies have an additional fee for this function.