Any active domain that is added in a hosting account includes a number of records that 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 certain records set through the hosting company like A (IP address of the website/server), MX (e mail server), SRV (using a domain address for non-website purposes on a specific port number such as Voice-over-IP), and many others. By having access to these records and by having the capacity to modify them, you could have your site with company A and your e-mails with company B, for illustration. This gives you more flexibility as you can choose the best provider for every single one of these services and you don't need to stick with a single company. After you pick 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.