Any active domain name that's added in a hosting account includes a variety of records that enable it to work properly. In the general case, the name servers (NS records) show the location where the domain name 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 (e mail server), SRV (using an Internet domain for non-website purposes on a specific port number like Voice-over-IP), and many others. By having access to these records and by having the capacity to modify them, you can have your site with company A and your emails with company B, for illustration. This gives you more flexibility as you can choose the best provider for each of these services and you don't need to stick to one company. As soon as you choose a hosting company, you have to make sure that you can access all DNS records freely because some companies have an extra fee for this feature.