Any active domain that's added in a hosting account has a number of records that allow it to function properly. In the typical case, the name servers (NS records) show the location where the domain name is hosted and they are 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 (e mail server), SRV (using a domain for non-website purposes on a certain port number like Voice-over-IP), and many others. By having access to these records and by having the ability to modify them, you can have your web site with company A and your emails with company B, for illustration. This offers you more overall flexibility because you can select the best provider for every single one of these services and you do not have to stick to one company. When you pick a hosting company, you have to make certain that you have access to all DNS records freely because some companies have an extra fee for this function.