Any active domain which is added in a hosting account features a number of records that allow it to perform properly. In the general 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 certain records created through the hosting company like A (IP address of the website/server), MX (email server), SRV (using a domain name for non-website purposes on a specific port number like Voice-over-IP), etc. By having access to these records and by being in a position to edit 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 since you can pick the best provider for every one of these services and you do not have to stick with a single company. When you select a hosting company, you have to make certain that you can access all DNS records freely since some companies have an additional fee for this feature.