Any active domain name which is added in a hosting account has a variety of records which allow it to perform properly. In the typical case, the name servers (NS records) show where the domain name is hosted and they're set on the side of the registrar company, but on the lower level there are particular records set through the hosting company like A (IP address of the website/server), MX (email server), SRV (using an Internet domain for non-website purposes on a certain port number like Voice-over-IP), etc. By having access to these records and by being able to modify them, you can have your web site with company A and your e-mails with company B, for instance. This offers you more flexibility since you can select the best provider for every single one of these services and you don't need to stick to one company. When you select a hosting company, you have to make sure that you have access to all DNS records freely as some providers have an additional fee for this function.