Any active domain that is added in a hosting account includes a variety of records that allow it to work properly. In the typical 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 like A (IP address of the website/server), MX (email server), SRV (using a domain address for non-website purposes on a certain port number such as Voice-over-IP), etc. By having access to these records and by being in a position to edit them, you can have your website with company A and your e-mails with company B, for illustration. This offers you more flexibility because you can select the best provider for every single one of these services and you don't have to stick with one company. As soon as you select a hosting company, you have to make sure that you have access to all DNS records freely as some companies have an additional fee for this function.