Any active domain that 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're 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 (employing a domain name for non-website purposes on a particular port number like Voice-over-IP), etc. With access to these records and by being able to edit them, you could have your site with company A and your e-mails with company B, for example. This offers you more overall flexibility because you can pick the best provider for every single one of these services and you do not have to stick with one company. As soon as you select a hosting company, you have to make certain that you can access all DNS records freely because some providers have an extra fee for this feature.