Any active domain name that is added in a hosting account has a number of records that allow it to perform properly. In the standard case, the name servers (NS records) show where the Internet domain is hosted and they're set on the side of the registrar company, but on the lower level there are certain records created through the hosting company like A (IP address of the website/server), MX (e-mail server), SRV (employing a domain address for non-website purposes on a certain port number like Voice-over-IP), and many others. Having access to these records and by having the ability to edit them, you could have your site with company A and your e-mail messages with company B, for illustration. This offers you more independence as you can pick the best provider for every single one of these services and you don't have to stick to a single company. When you pick a hosting company, you have to make sure that you can access all DNS records freely since some companies have an additional fee for this feature.