Any active domain which is added in a hosting account features a number of records which allow it to function properly. In the general case, the name servers (NS records) show where the Internet domain is hosted and they're set on the side of the registrar company, but on the lower level there are certain records set through the hosting company such as A (IP address of the website/server), MX (e mail server), SRV (employing a domain name for non-website purposes on a certain port number such as Voice-over-IP), etc. By having access to these records and by being able to modify them, you could have your site with company A and your e-mails with company B, for illustration. This gives you more overall flexibility as you can choose the best provider for every one of these services and you don't have to stick with one company. As soon as you select a hosting company, you have to make certain that you can access all DNS records freely as some companies have an additional fee for this feature.