Any active domain name which is added in a hosting account includes a number of records that permit it to function properly. In the typical 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 such as A (IP address of the website/server), MX (e-mail server), SRV (using a domain address for non-website purposes on a certain port number like Voice-over-IP), etc. By having access to these records and by having the capacity to edit them, you may have your website with company A and your e-mails with company B, for instance. This offers you more freedom because you can choose the best provider for every single one of these services and you don't have to stick with a single company. As soon as you pick a hosting company, you have to make sure that you have access to all DNS records freely because some providers have an extra fee for this feature.