Any active domain that is added in a hosting account features a variety of records that allow it to perform 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 address for non-website purposes on a particular port number like Voice-over-IP), etc. With access to these records and by having the ability to modify them, you can have your web site with company A and your emails with company B, for illustration. This gives you more flexibility as you can select the best provider for every single one of these services and you don't need to stick to 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 extra fee for this feature.