Any active domain that's added in a hosting account features a number of records that permit it to work properly. In the general case, the name servers (NS records) show the location where the domain name 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 (employing an Internet domain for non-website purposes on a certain port number such as Voice-over-IP), etc. By having access to these records and by being in a position to edit them, you can have your site with company A and your e-mail messages with company B, for instance. This gives you more overall flexibility as you can select the best provider for each of these services and you do not have to stick with one company. As soon as you pick a hosting company, you have to make certain that you have access to all DNS records freely as some providers have an extra fee for this feature.