Any active domain that is added in a hosting account has a number of records that enable it to perform properly. In the general 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 certain records set 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 like Voice-over-IP), and so on. With access to these records and by having the capacity to edit them, you can have your website with company A and your emails with company B, for instance. This gives you more independence because you can choose the best provider for every single one of these services and you don't have to stick with a single company. Whenever you select a hosting company, you have to make sure that you have access to all DNS records freely since some providers have an additional fee for this function.