Any active domain name which is added in a hosting account features a number of records that allow it to work properly. In the standard case, the name servers (NS records) show where the domain is hosted and they are set on the side of the registrar company, but on the lower level there are certain records created through the hosting company such as A (IP address of the website/server), MX (e mail server), SRV (employing a domain for non-website purposes on a specific port number such as Voice-over-IP), and so on. Having access to these records and by being able to modify them, you could have your website with company A and your e-mail messages with company B, for instance. This gives you more overall flexibility as you can pick the best provider for every single one of these services and you don't have to stick with only one company. Whenever you choose a hosting company, you have to make sure that you can access all DNS records freely because some companies have an extra fee for this function.