Any active domain that is added in a hosting account includes a variety of records that allow it to perform properly. In the standard case, the name servers (NS records) show the location where the Internet domain is hosted and they are set on the side of the registrar company, but on the lower level there are specific records set through the hosting company such as A (IP address of the website/server), MX (e-mail server), SRV (using a domain for non-website purposes on a certain port number like Voice-over-IP), etc. By having access to these records and by having the ability to modify them, you could have your site with company A and your e-mails with company B, for instance. This offers you more freedom as you can select the best provider for each of these services and you don't have to stick with one company. After you choose a hosting company, you have to make sure that you have access to all DNS records freely since some providers have an extra fee for this function.