Any active domain which is added in a hosting account includes a variety of records that permit it to work properly. In the typical case, the name servers (NS records) show 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 created through the hosting company like A (IP address of the website/server), MX (email server), SRV (using a domain address for non-website purposes on a specific port number such as Voice-over-IP), etc. With access to these records and by being able to modify them, you could have your web site with company A and your emails with company B, for example. This offers you more independence as you can choose the best provider for every one of these services and you do not have to stick with one company. Whenever you select a hosting company, you have to make sure that you can access all DNS records freely since some providers have an extra fee for this feature.