Any active domain which is added in a hosting account features a number of records which allow it to perform properly. In the typical 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 specific records set through the hosting company like A (IP address of the website/server), MX (email server), SRV (using an Internet domain for non-website purposes on a specific port number such as Voice-over-IP), and so on. With access to these records and by having the ability to modify them, you could have your website with company A and your e-mails with company B, for example. This gives you more independence since you can select the best provider for every single one of these services and you don't need to stick with a single company. When you pick a hosting company, you have to make certain that you have access to all DNS records freely since some companies have an extra fee for this function.