Any active domain that's added in a hosting account has a variety of records which allow it to perform properly. In the general 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 specific records set through the hosting company such as A (IP address of the website/server), MX (email server), SRV (employing a domain address for non-website purposes on a specific port number like Voice-over-IP), etc. With access to these records and by having the ability to modify them, you can have your site with company A and your e-mail messages with company B, for instance. This offers you more independence since you can choose the best provider for every single one of these services and you do not 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 feature.