Any active domain that's added in a hosting account has a number of records that allow it to work properly. In the standard 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 a domain for non-website purposes on a particular port number such as Voice-over-IP), and so on. With access to these records and by being able to edit them, you could have your site with company A and your e-mails with company B, for instance. This offers you more flexibility as you can pick the best provider for each of these services and you don't need to stick with a single company. When you choose a hosting company, you have to make sure that you have access to all DNS records freely since some providers have an additional fee for this function.