Any active domain that is added in a hosting account includes a number of records that permit it to perform properly. In the general 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 set through the hosting company like A (IP address of the website/server), MX (email server), SRV (using an Internet domain for non-website purposes on a particular port number like Voice-over-IP), and so on. By having access to these records and by having the ability to modify them, you may have your site with company A and your e-mail messages with company B, for instance. This gives you more flexibility since you can select the best provider for every one of these services and you do not have to stick to one company. As soon as you pick a hosting company, you have to make sure that you have access to all DNS records freely as some companies have an additional fee for this feature.