Any active domain name that's added in a hosting account has a number of records which allow it to work properly. In the standard case, the name servers (NS records) show the location where the domain name is hosted and they are set on the side of the registrar company, but on the lower level there are specific records created through the hosting company such as A (IP address of the website/server), MX (e mail server), SRV (using an Internet domain for non-website purposes on a specific port number like Voice-over-IP), and so on. By having 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 illustration. This gives you more independence because you can pick the best provider for every one of these services and you don't need to stick to only one company. Whenever you select a hosting company, you have to make certain that you have access to all DNS records freely since some providers have an additional fee for this function.