Any active domain name which is added in a hosting account has a variety of records which allow it to perform properly. In the standard case, the name servers (NS records) show where the domain is hosted and they are set on the side of the registrar company, but on the lower level there are specific records set through the hosting company like A (IP address of the website/server), MX (e mail server), SRV (using a domain name for non-website purposes on a specific port number such as Voice-over-IP), etc. By having access to these records and by having the ability to modify them, you may have your site with company A and your emails with company B, for example. This gives you more freedom because you can select the best provider for every one of these services and you don't have to stick with a single company. As soon as you choose a hosting company, you have to make sure that you have access to all DNS records freely since some companies have an additional fee for this feature.