Any active domain that's added in a hosting account features a variety of records that permit it to function properly. In the standard case, the name servers (NS records) show the location where the Internet 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 like A (IP address of the website/server), MX (email server), SRV (using a domain address 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 capacity to modify them, you may have your site with company A and your e-mail messages with company B, for example. This offers you more flexibility as you can pick the best provider for every one of these services and you don't need to stick to a single company. When you choose a hosting company, you have to make sure that you have access to all DNS records freely because some providers have an additional fee for this feature.