Any active domain that's added in a hosting account features a variety of records that allow it to perform properly. In the standard case, the name servers (NS records) show where the Internet domain is hosted and they are 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 (e mail server), SRV (using a domain name for non-website purposes on a certain port number such as Voice-over-IP), etc. Having access to these records and by being in a position to modify them, you may have your site with company A and your emails with company B, for example. This gives you more overall flexibility since you can pick the best provider for each of these services and you don't have to stick to a single company. Whenever you select a hosting company, you have to make sure that you have access to all DNS records freely because some providers have an additional fee for this feature.