Any active domain name that is added in a hosting account features a number of records which allow it to function properly. In the typical case, the name servers (NS records) show the location where the domain name is hosted and they're 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 (using an Internet domain for non-website purposes on a certain port number such as Voice-over-IP), and many others. Having access to these records and by being able to modify them, you can have your website with company A and your emails with company B, for instance. This offers you more flexibility since you can pick the best provider for every one of these services and you don't need to stick to one company. As soon as you select a hosting company, you have to make sure that you have access to all DNS records freely since some companies have an additional fee for this function.