Any active domain name which is added in a hosting account features a variety of records which allow it to perform properly. In the common 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 certain records set through the hosting company such as A (IP address of the website/server), MX (email server), SRV (employing an Internet domain for non-website purposes on a particular port number like Voice-over-IP), and many others. Having access to these records and by being able to edit them, you could have your website with company A and your e-mails with company B, for illustration. This offers you more independence because you can choose the best provider for each of these services and you do not have to stick to one company. After you choose a hosting company, you have to make certain that you have access to all DNS records freely as some providers have an extra fee for this feature.
