Any active domain that is added in a hosting account features a number of records that enable it to function properly. In the standard case, the name servers (NS records) show the location where the domain is hosted and they're set on the side of the registrar company, but on the lower level there are certain records created through the hosting company like A (IP address of the website/server), MX (email server), SRV (using a domain name for non-website purposes on a certain port number such as Voice-over-IP), and so on. Having access to these records and by having the capacity to modify them, you may have your web site with company A and your e-mails with company B, for example. This offers you more freedom because you can pick the best provider for every single one of these services and you don't need to stick to one company. When you choose a hosting company, you have to make certain that you can access all DNS records freely since some companies have an extra fee for this function.