Any active domain that is added in a hosting account has a variety of records which allow it to function properly. In the common 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 particular records created through the hosting company like A (IP address of the website/server), MX (e-mail server), SRV (using a domain for non-website purposes on a specific port number like Voice-over-IP), and so on. By having access to these records and by being in a position to modify them, you could have your website with company A and your e-mails with company B, for illustration. This gives you more independence as you can choose the best provider for every single one of these services and you do not have to stick to one company. Whenever you pick a hosting company, you have to make certain that you can access all DNS records freely because some providers have an additional fee for this function.