Any active domain name which is added in a hosting account features a number of records that allow it to function properly. In the standard case, the name servers (NS records) show the location where the Internet domain is hosted and they're set on the side of the registrar company, but on the lower level there are certain records set through the hosting company such as A (IP address of the website/server), MX (email server), SRV (employing a domain address for non-website purposes on a specific port number such as Voice-over-IP), and so on. Having access to these records and by having the ability to edit them, you can have your site with company A and your emails with company B, for example. This gives you more flexibility as you can select the best provider for each of these services and you don't have to stick to one company. As soon as you pick a hosting company, you have to make sure that you have access to all DNS records freely as some companies have an additional fee for this feature.