Any active domain that is added in a hosting account has a variety of records that permit 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 specific records set 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 many others. By having access to these records and by having the ability to edit them, you could have your site with company A and your e-mails with company B, for instance. This gives you more freedom as you can select the best provider for every one of these services and you don't have to stick with only one company. After you choose a hosting company, you have to make sure that you can access all DNS records freely because some providers have an extra fee for this function.