Any active domain name that's added in a hosting account features a number of records which allow it to function properly. In the typical case, the name servers (NS records) show where the domain name is hosted and they're set on the side of the registrar company, but on the lower level there are certain records set through the hosting company such as A (IP address of the website/server), MX (email server), SRV (employing an Internet domain for non-website purposes on a specific port number such as Voice-over-IP), etc. With access to these records and by being in a position to modify them, you may have your site with company A and your e-mails with company B, for example. This offers you more freedom as you can choose the best provider for every single one of these services and you do not have to stick to only one company. As soon as you select a hosting company, you have to make sure that you have access to all DNS records freely since some providers have an extra fee for this function.