Any active domain name that is added in a hosting account includes a number of records that enable it to work properly. In the common case, the name servers (NS records) show the location where the domain name 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 such as A (IP address of the website/server), MX (e-mail server), SRV (employing a domain for non-website purposes on a specific port number like Voice-over-IP), and many others. With access to these records and by being able to modify them, you can have your web site with company A and your emails with company B, for example. This gives you more independence as you can pick the best provider for every one of these services and you don't have to stick to one company. When you choose a hosting company, you have to make certain that you can access all DNS records freely as some companies have an additional fee for this function.