Any active domain that's added in a hosting account includes a variety of records that permit it to work properly. In the common case, the name servers (NS records) show where the domain 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 like A (IP address of the website/server), MX (email server), SRV (using a domain address for non-website purposes on a particular port number such as Voice-over-IP), and so on. Having access to these records and by being able to modify them, you could have your web site with company A and your emails with company B, for example. This offers you more freedom because you can pick the best provider for every one of these services and you do not have to stick to only one company. As soon as you pick a hosting company, you have to make certain that you have access to all DNS records freely since some companies have an extra fee for this feature.