Any active domain name which is added in a hosting account features a variety of records which allow it to perform properly. In the typical 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 certain records set 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. By having access to these records and by having the ability to modify them, you can have your site with company A and your e-mail messages with company B, for example. This gives you more overall flexibility since you can select the best provider for each of these services and you don't need to stick with only one company. As soon as you pick a hosting company, you have to make sure that you have access to all DNS records freely since some providers have an extra fee for this function.