Any active domain that is added in a hosting account includes a number of records that enable it to function properly. In the typical case, the name servers (NS records) show the location where the Internet domain 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 (employing a domain for non-website purposes on a specific port number such as Voice-over-IP), and many others. With access to these records and by having the ability to modify them, you could have your site with company A and your emails with company B, for instance. This gives you more overall flexibility because you can select the best provider for every one of these services and you do not have to stick to one company. Whenever you choose a hosting company, you have to make sure that you can access all DNS records freely as some providers have an extra fee for this function.