Any active domain that is added in a hosting account features a variety of records which allow it to perform properly. In the standard 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 set through the hosting company such as A (IP address of the website/server), MX (email server), SRV (using a domain name for non-website purposes on a particular port number like Voice-over-IP), etc. With access to these records and by being able to modify them, you could have your web site with company A and your e-mail messages with company B, for instance. This offers you more flexibility since you can pick the best provider for every one of these services and you don't need to stick with only one company. Whenever you pick a hosting company, you have to make certain that you can access all DNS records freely because some companies have an additional fee for this feature.