Any active domain that's added in a hosting account includes a number of records that allow it to function properly. In the typical case, the name servers (NS records) show where the Internet 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 (e mail server), SRV (using a domain name for non-website purposes on a certain port number like Voice-over-IP), etc. Having access to these records and by being able to modify them, you may have your site with company A and your e-mails with company B, for illustration. This offers you more freedom as you can select the best provider for each of these services and you don't have to stick with one company. As soon as you select a hosting company, you have to make sure that you have access to all DNS records freely because some providers have an extra fee for this function.