Any active domain name that's added in a hosting account has a number of records which allow it to work properly. In the general case, the name servers (NS records) show where the domain name is hosted and they are set on the side of the registrar company, but on the lower level there are certain records created 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 specific port number like Voice-over-IP), and many others. Having access to these records and by having the capacity to modify them, you could have your web site with company A and your emails with company B, for illustration. 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. Whenever you pick a hosting company, you have to make certain that you can access all DNS records freely since some companies have an extra fee for this feature.