Any active domain name which is added in a hosting account features a variety of records which allow it to function properly. In the general case, the name servers (NS records) show 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 set through the hosting company such as A (IP address of the website/server), MX (e mail server), SRV (using a domain address for non-website purposes on a certain port number such as Voice-over-IP), etc. By having access to these records and by having the ability to modify them, you could have your site with company A and your e-mail messages with company B, for illustration. This offers you more freedom because you can select the best provider for every one of these services and you don't need to stick to a single company. Whenever you select a hosting company, you have to make certain that you have access to all DNS records freely as some companies have an additional fee for this function.