Any active domain name which is added in a hosting account has a number of records that allow it to perform properly. In the common 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 certain records set through the hosting company like A (IP address of the website/server), MX (e-mail server), SRV (using an Internet domain for non-website purposes on a particular port number like Voice-over-IP), etc. Having access to these records and by being able to edit them, you can have your web site with company A and your emails with company B, for illustration. This offers you more freedom as you can pick the best provider for each of these services and you don't have to stick with a single company. Whenever you choose a hosting company, you have to make sure that you can access all DNS records freely because some companies have an additional fee for this function.