Any active domain name that is added in a hosting account has a variety of records which 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 particular records created through the hosting company such as A (IP address of the website/server), MX (e mail server), SRV (using a domain for non-website purposes on a particular port number like Voice-over-IP), and many others. With access to these records and by being able to edit them, you can have your website with company A and your e-mail messages with company B, for instance. This gives you more independence since you can choose the best provider for every one of these services and you don't have to stick with one company. Whenever you select a hosting company, you have to make sure that you have access to all DNS records freely since some companies have an additional fee for this function.