Any active domain name that's added in a hosting account includes a number of records that permit it to work properly. In the common case, the name servers (NS records) show where the domain is hosted and they're set on the side of the registrar company, but on the lower level there are particular records created through the hosting company like A (IP address of the website/server), MX (e mail server), SRV (employing a domain name for non-website purposes on a particular port number like Voice-over-IP), and so on. By having access to these records and by being able to modify them, you can have your site with company A and your emails with company B, for illustration. This gives you more freedom as you can choose the best provider for each of these services and you do not have to stick to a single company. After you pick a hosting company, you have to make sure that you can access all DNS records freely since some providers have an additional fee for this function.