Any active domain which is added in a hosting account features a variety of records which allow it to perform properly. In the general case, the name servers (NS records) show 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 set through the hosting company such as A (IP address of the website/server), MX (e-mail server), SRV (using a domain address for non-website purposes on a particular port number like Voice-over-IP), etc. Having access to these records and by having the ability to edit them, you may have your web site with company A and your emails with company B, for illustration. This gives you more flexibility since you can select the best provider for every single one of these services and you do not have to stick with one company. When you select 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.