Any active domain name that is added in a hosting account features a variety of records which allow it to work properly. In the general case, the name servers (NS records) show the location where the domain name is hosted and they are set on the side of the registrar company, but on the lower level there are particular records created through the hosting company such as A (IP address of the website/server), MX (e-mail server), SRV (using a domain for non-website purposes on a particular port number like Voice-over-IP), and many others. With access to these records and by being able to edit them, you could have your site with company A and your emails with company B, for illustration. This gives you more independence as you can choose the best provider for every one of these services and you do not have to stick with only one company. As soon as you choose 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 feature.