Any active domain name that is added in a hosting account includes a variety of records that enable it to perform properly. In the common case, the name servers (NS records) show 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 (email server), SRV (employing a domain for non-website purposes on a specific port number like Voice-over-IP), etc. By having access to these records and by having the capacity to modify them, you can have your web site with company A and your e-mails with company B, for illustration. This gives you more overall flexibility as you can choose the best provider for every single one of these services and you don't need to stick to one company. When you choose a hosting company, you have to make sure that you have access to all DNS records freely since some providers have an additional fee for this feature.