Any active domain name that's added in a hosting account has a number 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 are set on the side of the registrar company, but on the lower level there are particular records created through the hosting company such as 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. With access to these records and by having the capacity to modify them, you could have your site with company A and your emails with company B, for instance. This gives you more freedom as you can pick the best provider for every single one of these services and you don't need to stick with only one company. After you choose a hosting company, you have to make sure that you have access to all DNS records freely because some companies have an extra fee for this function.