Any active domain name that is added in a hosting account features a number of records which allow it to work properly. In the typical case, the name servers (NS records) show where the domain is hosted and they are set on the side of the registrar company, but on the lower level there are specific 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 certain port number like Voice-over-IP), and many others. Having access to these records and by being in a position to edit them, you could have your site with company A and your e-mails with company B, for example. This offers you more independence as you can select the best provider for each of these services and you do not have to stick to a single company. As soon as 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 function.