Any active domain that's added in a hosting account includes a number of records that permit it to work properly. In the standard case, the name servers (NS records) show where the Internet domain is hosted and they're set on the side of the registrar company, but on the lower level there are specific records set through the hosting company like A (IP address of the website/server), MX (email server), SRV (using a domain for non-website purposes on a particular port number such as Voice-over-IP), etc. With access to these records and by having the capacity to modify them, you could have your site with company A and your e-mail messages with company B, for example. This gives you more overall flexibility as you can pick the best provider for each of these services and you don't have to stick to a single company. As soon as you pick a hosting company, you have to make certain that you have access to all DNS records freely because some companies have an additional fee for this function.