Any active domain that's added in a hosting account features a number of records that permit it to work properly. In the standard case, the name servers (NS records) show the location 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 such as A (IP address of the website/server), MX (e-mail server), SRV (employing a domain address for non-website purposes on a particular port number such as Voice-over-IP), and so on. With access to these records and by having the capacity to modify them, you could have your website with company A and your emails with company B, for instance. This gives you more freedom since you can pick the best provider for every one of these services and you don't need to stick to a single company. As soon as you pick a hosting company, you have to make certain that you have access to all DNS records freely as some providers have an extra fee for this function.