Any active domain name that's added in a hosting account has a variety of records that permit it to perform properly. In the typical case, the name servers (NS records) show where the domain is hosted and they are set on the side of the registrar company, but on the lower level there are specific records set 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 so on. Having 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 instance. This offers you more freedom because you can pick the best provider for every single one of these services and you do not have to stick with one company. As soon as you pick a hosting company, you have to make sure that you can access all DNS records freely as some providers have an extra fee for this function.