Any active domain that is added in a hosting account includes a variety of records which allow it to perform properly. In the standard case, the name servers (NS records) show where the domain name is hosted and they're 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 (employing a domain name for non-website purposes on a specific port number like Voice-over-IP), etc. Having access to these records and by being able to edit them, you may have your website with company A and your emails with company B, for instance. This gives you more flexibility as you can pick the best provider for each of these services and you don't have to stick with one company. After you pick a hosting company, you have to make sure that you can access all DNS records freely as some companies have an additional fee for this feature.