Any active domain that is added in a hosting account has a variety of records that permit it to work properly. In the standard case, the name servers (NS records) show where the Internet domain is hosted and they are 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 an Internet domain for non-website purposes on a specific port number like Voice-over-IP), etc. By having access to these records and by having the ability to modify them, you can have your website with company A and your e-mails with company B, for instance. This offers you more overall flexibility as you can select the best provider for every single one of these services and you don't have to stick with one company. When you choose a hosting company, you have to make sure that you have access to all DNS records freely since some companies have an additional fee for this feature.