Any active domain that's added in a hosting account has a variety of records that enable it to work properly. In the standard 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 specific records created through the hosting company like A (IP address of the website/server), MX (e mail server), SRV (using a domain for non-website purposes on a certain port number like Voice-over-IP), and so on. Having access to these records and by being in a position to edit them, you can have your website with company A and your e-mails with company B, for instance. This offers you more overall flexibility as 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 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.