Any active domain that's added in a hosting account includes a variety of records that enable it to function properly. In the common case, the name servers (NS records) show where the Internet domain is hosted and they are set on the side of the registrar company, but on the lower level there are certain records created through the hosting company such as A (IP address of the website/server), MX (email server), SRV (employing an Internet domain 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 illustration. This gives you more flexibility as you can choose the best provider for each of these services and you don't have to stick with a single company. After you choose 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 function.