Any active domain name which is added in a hosting account includes a variety of records that allow it to work properly. In the standard case, the name servers (NS records) show the location where the domain name 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 name for non-website purposes on a certain port number such as Voice-over-IP), and so on. Having access to these records and by having the ability to modify them, you may have your web site with company A and your emails with company B, for example. This offers you more flexibility because you can pick the best provider for every one of these services and you don't have to stick with a single company. When you pick a hosting company, you have to make sure that you can access all DNS records freely because some providers have an additional fee for this function.