Any active domain which is added in a hosting account features a variety of records which allow it to function 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 set through the hosting company such as A (IP address of the website/server), MX (email server), SRV (employing a domain name for non-website purposes on a certain port number like Voice-over-IP), and so on. With access to these records and by having the ability to edit them, you could have your website with company A and your emails with company B, for example. This offers you more independence because you can pick the best provider for every one of these services and you don't need to stick with a single company. Whenever you pick a hosting company, you have to make sure that you can access all DNS records freely because some providers have an additional fee for this function.