Any active domain which is added in a hosting account includes a number of records that enable it to work properly. In the general 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 particular records created through the hosting company like A (IP address of the website/server), MX (e-mail server), SRV (employing a domain for non-website purposes on a specific port number like Voice-over-IP), and many others. With access to these records and by being able to modify them, you can have your website with company A and your e-mails with company B, for example. This offers you more independence because you can select the best provider for every one of these services and you don't need to stick to one company. When you pick a hosting company, you have to make certain that you can access all DNS records freely because some providers have an additional fee for this function.