Any active domain which is added in a hosting account has a number of records that allow it to work properly. In the general 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 certain records created through the hosting company such as A (IP address of the website/server), MX (e mail server), SRV (employing a domain address for non-website purposes on a particular port number like Voice-over-IP), and many others. With access to these records and by being able to modify them, you could have your site with company A and your emails with company B, for instance. This gives you more flexibility as you can pick the best provider for each of these services and you don't have to stick to one company. As soon as you select a hosting company, you have to make sure that you have access to all DNS records freely as some providers have an extra fee for this function.