Any active domain which is added in a hosting account includes a number of records that allow it to work properly. In the common case, the name servers (NS records) show the location where the domain name is hosted and they're set on the side of the registrar company, but on the lower level there are specific records set through the hosting company such as A (IP address of the website/server), MX (e mail server), SRV (employing an Internet domain for non-website purposes on a certain port number like Voice-over-IP), etc. With access to these records and by having the capacity to edit them, you could have your web site with company A and your emails with company B, for instance. This gives you more flexibility because you can choose the best provider for every single one of these services and you do not have to stick with one company. After you choose a hosting company, you have to make certain that you can access all DNS records freely since some providers have an extra fee for this function.