Any active domain that is added in a hosting account has a number 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 particular records created through the hosting company like A (IP address of the website/server), MX (e mail server), SRV (using a domain address for non-website purposes on a particular port number such as Voice-over-IP), and many others. Having access to these records and by having the ability to modify them, you may have your site with company A and your e-mail messages with company B, for illustration. This offers you more overall flexibility since you can pick the best provider for every one of these services and you don't need to stick to a single company. As soon as you select a hosting company, you have to make sure that you have access to all DNS records freely as some providers have an extra fee for this feature.