Any active domain name that is added in a hosting account features a number of records that enable it to perform properly. In the common case, the name servers (NS records) show the location where the domain is hosted and they are 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 (e-mail server), SRV (using an Internet domain 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 ability to modify them, you may have your website with company A and your emails with company B, for example. This gives you more overall flexibility as you can choose the best provider for each of these services and you do not have to stick to a single company. Whenever 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.