Any active domain which is added in a hosting account includes a number of records which allow it to function properly. In the standard case, the name servers (NS records) show where the domain name is hosted and they are set on the side of the registrar company, but on the lower level there are certain records created through the hosting company such as A (IP address of the website/server), MX (email server), SRV (employing an Internet domain for non-website purposes on a specific port number such as Voice-over-IP), and many others. With access to these records and by having the capacity to modify them, you may have your site with company A and your e-mails with company B, for example. This offers you more flexibility because you can pick the best provider for each of these services and you don't have to stick with a single company. As soon as you choose a hosting company, you have to make sure that you can access all DNS records freely as some providers have an additional fee for this feature.