Any active domain which is added in a hosting account includes a number of records which allow it to perform properly. In the standard case, the name servers (NS records) show the location where the domain name 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 like A (IP address of the website/server), MX (email server), SRV (employing a domain for non-website purposes on a certain port number like Voice-over-IP), and so on. By having access to these records and by having the ability to edit them, you could have your site with company A and your emails with company B, for example. This gives you more independence since you can choose the best provider for every one of these services and you don't have to stick with one company. When you pick a hosting company, you have to make sure that you can access all DNS records freely since some providers have an extra fee for this function.