Any active domain name that's added in a hosting account has a variety of records that enable it to function properly. In the general case, the name servers (NS records) show 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 created through the hosting company such as A (IP address of the website/server), MX (email server), SRV (employing a domain name for non-website purposes on a specific port number like Voice-over-IP), etc. Having access to these records and by being in a position to edit them, you could have your site with company A and your e-mail messages with company B, for instance. This offers you more freedom as you can select the best provider for every single one of these services and you don't have to stick with a single company. As soon as you select a hosting company, you have to make sure that you can access all DNS records freely as some providers have an additional fee for this function.