Any active domain name that is added in a hosting account has a variety of records that enable it to function properly. In the typical case, the name servers (NS records) show the location where the domain is hosted and they're set on the side of the registrar company, but on the lower level there are specific records created through the hosting company such as A (IP address of the website/server), MX (email server), SRV (using a domain address for non-website purposes on a particular port number such as Voice-over-IP), and many others. With access to these records and by being able to edit them, you may have your site with company A and your e-mails with company B, for illustration. This offers you more overall flexibility since you can pick the best provider for every single one of these services and you don't need to stick with a single company. Whenever you choose a hosting company, you have to make sure that you have access to all DNS records freely as some providers have an extra fee for this feature.