Any active domain that is added in a hosting account features a variety of records that allow it to work properly. In the typical case, the name servers (NS records) show the location where the Internet domain is hosted and they're 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 (email server), SRV (employing an Internet domain for non-website purposes on a particular port number such as Voice-over-IP), etc. Having access to these records and by being able to modify them, you may have your web site with company A and your e-mail messages with company B, for instance. This gives you more overall flexibility as you can choose the best provider for every one of these services and you don't need to stick to only one company. As soon as you pick a hosting company, you have to make sure that you have access to all DNS records freely because some providers have an extra fee for this function.