Any active domain which is added in a hosting account features a number of records that enable it to work properly. In the general case, the name servers (NS records) show the location where the domain name is hosted and they're 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 (using an Internet domain for non-website purposes on a certain port number like Voice-over-IP), and many others. With access to these records and by being in a position 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 overall flexibility since you can select the best provider for every single one of these services and you don't need to stick with only one company. As soon as you pick a hosting company, you have to make certain that you can access all DNS records freely as some providers have an additional fee for this function.