Any active domain 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 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 like A (IP address of the website/server), MX (e-mail server), SRV (using an Internet domain for non-website purposes on a particular port number such as Voice-over-IP), etc. By having access to these records and by having the capacity to modify them, you can have your site with company A and your e-mails with company B, for instance. This offers you more independence because you can choose the best provider for each of these services and you don't need to stick with one company. Whenever you pick 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.