Any active domain that's added in a hosting account has a variety of records that permit it to work properly. In the standard case, the name servers (NS records) show the location where the Internet domain 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 (e-mail server), SRV (using a domain for non-website purposes on a specific port number like Voice-over-IP), and many others. With access to these records and by having the ability to modify them, you can have your site with company A and your emails with company B, for instance. This offers you more flexibility because you can select the best provider for each of these services and you do not have to stick with one company. As soon as you select a hosting company, you have to make sure that you have access to all DNS records freely as some providers have an additional fee for this feature.