Any active domain that's added in a hosting account features a variety of records that permit it to perform properly. In the typical case, the name servers (NS records) show the location where the domain is hosted and they're set on the side of the registrar company, but on the lower level there are specific 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 so on. Having access to these records and by having the ability to modify them, you can have your web site with company A and your e-mails with company B, for illustration. This offers you more freedom as you can choose the best provider for each of these services and you don't need to stick with only one company. As soon as you choose a hosting company, you have to make sure that you can access all DNS records freely because some providers have an additional fee for this feature.