Any active domain that is added in a hosting account includes a number of records which allow it to function properly. In the standard case, the name servers (NS records) show where the domain name 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 such as A (IP address of the website/server), MX (email server), SRV (using an Internet domain for non-website purposes on a certain port number like Voice-over-IP), and many others. Having access to these records and by being able to modify them, you can have your site with company A and your e-mail messages with company B, for illustration. This offers you more independence since you can select the best provider for each of these services and you don't have to stick to one company. As soon as you select a hosting company, you have to make certain that you can access all DNS records freely as some providers have an extra fee for this feature.