Any active domain which is added in a hosting account features a number of records that enable it to function properly. In the standard case, the name servers (NS records) show the location where the Internet domain is hosted and they're set on the side of the registrar company, but on the lower level there are specific 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 particular port number like Voice-over-IP), and many others. Having access to these records and by being able to modify them, you can have your website with company A and your e-mails with company B, for illustration. This gives you more freedom since you can choose the best provider for every one of these services and you don't have to stick with a single company. When you choose a hosting company, you have to make sure that you can access all DNS records freely because some providers have an additional fee for this function.