Any active domain name that is added in a hosting account features a number of records which allow it to work properly. In the typical case, the name servers (NS records) show where the domain 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 (e-mail server), SRV (employing a domain address for non-website purposes on a particular port number such as Voice-over-IP), and so on. With access to these records and by being able to edit them, you can have your website with company A and your e-mails with company B, for illustration. This gives you more flexibility as you can pick the best provider for every one of these services and you do not have to stick with one company. As soon as you select 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.