Any active domain that is added in a hosting account features a variety of records which allow it to function properly. In the standard case, the name servers (NS records) show the location where the domain 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 (employing a domain for non-website purposes on a specific port number such as Voice-over-IP), etc. With 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 offers you more freedom as you can select the best provider for every one of these services and you do not have to stick to only one company. Whenever you pick a hosting company, you have to make certain that you can access all DNS records freely as some providers have an extra fee for this feature.