Any active domain name that's added in a hosting account includes a number of records which allow it to perform properly. In the common case, the name servers (NS records) show where the domain is hosted and they are 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 (e mail server), SRV (employing a domain for non-website purposes on a certain port number like Voice-over-IP), and so on. Having access to these records and by having the ability to edit them, you could have your website with company A and your emails with company B, for instance. This gives you more independence as you can select the best provider for each of these services and you don't need to stick to only one company. When you choose a hosting company, you have to make certain that you can access all DNS records freely since some providers have an additional fee for this function.