Any active domain name that is added in a hosting account includes a number of records that enable it to function properly. In the typical case, the name servers (NS records) show where the domain name is hosted and they're set on the side of the registrar company, but on the lower level there are particular records created through the hosting company like A (IP address of the website/server), MX (e mail server), SRV (using an Internet domain for non-website purposes on a specific port number like Voice-over-IP), and many others. With access to these records and by being able to modify them, you can have your web site with company A and your e-mails with company B, for example. This offers you more overall flexibility because you can choose the best provider for every one of these services and you don't need to stick to only one company. When you pick a hosting company, you have to make sure that you have access to all DNS records freely because some companies have an extra fee for this function.