Any active domain that's added in a hosting account has a variety of records that enable it to function properly. In the standard 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 such as A (IP address of the website/server), MX (e mail server), SRV (using a domain for non-website purposes on a specific port number such as Voice-over-IP), and many others. Having access to these records and by being able to modify them, you could have your site with company A and your e-mails with company B, for instance. This offers you more independence since you can select the best provider for every single one of these services and you don't need to stick to only one company. After you pick a hosting company, you have to make sure that you have access to all DNS records freely because some providers have an additional fee for this function.