Any active domain name that's added in a hosting account features a variety of records that allow it to function 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 specific records created through the hosting company like A (IP address of the website/server), MX (e mail server), SRV (employing a domain address for non-website purposes on a certain port number such as Voice-over-IP), etc. Having access to these records and by being in a position to modify them, you can have your website with company A and your e-mail messages with company B, for example. This offers you more freedom since you can pick the best provider for every single one of these services and you don't have to stick to a single company. As soon as you pick a hosting company, you have to make sure that you can access all DNS records freely because some providers have an extra fee for this feature.