Any active domain that is added in a hosting account features a variety of records which allow it to function properly. In the general 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 certain records created through the hosting company such as A (IP address of the website/server), MX (e mail server), SRV (using a domain for non-website purposes on a particular port number such as Voice-over-IP), etc. With access to these records and by being able to edit them, you may have your web site with company A and your e-mail messages with company B, for illustration. This gives you more flexibility because you can pick the best provider for every one of these services and you don't have to stick to a single company. After you pick a hosting company, you have to make sure that you have access to all DNS records freely since some companies have an additional fee for this function.