Any active domain name that's added in a hosting account includes a variety of records which allow it to function properly. In the common case, the name servers (NS records) show the location where the domain is hosted and they are set on the side of the registrar company, but on the lower level there are certain records set through the hosting company such as A (IP address of the website/server), MX (e-mail server), SRV (using a domain for non-website purposes on a specific port number such as Voice-over-IP), and many others. Having access to these records and by having the capacity to edit them, you could have your site with company A and your emails with company B, for example. This gives you more independence because you can pick the best provider for every one of these services and you do not have to stick to a single company. Whenever you choose a hosting company, you have to make certain that you can access all DNS records freely because some companies have an additional fee for this function.