Any active domain that's added in a hosting account has a number of records that allow it to perform properly. In the standard case, the name servers (NS records) show the location where the 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 a domain address for non-website purposes on a particular port number like Voice-over-IP), etc. Having access to these records and by being in a position to modify them, you can have your site with company A and your e-mails with company B, for instance. This gives you more flexibility as you can choose the best provider for each of these services and you don't need to stick to a single company. When you pick a hosting company, you have to make sure that you can access all DNS records freely because some providers have an extra fee for this feature.