Any active domain that's added in a hosting account includes a number of records that permit it to function properly. In the common case, the name servers (NS records) show where the domain is hosted and they are set on the side of the registrar company, but on the lower level there are specific records set through the hosting company such as A (IP address of the website/server), MX (e-mail server), SRV (using 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 being in a position to modify them, you may have your site with company A and your emails with company B, for example. This offers you more freedom since you can pick the best provider for each of these services and you don't need to stick with one company. As soon as you pick a hosting company, you have to make sure that you can access all DNS records freely because some companies have an extra fee for this function.