Any active domain name which is added in a hosting account includes a variety of records that allow it to function properly. In the standard case, the name servers (NS records) show where the domain is hosted and they are set on the side of the registrar company, but on the lower level there are certain records created through the hosting company like A (IP address of the website/server), MX (e-mail server), SRV (using a domain address for non-website purposes on a certain port number such as Voice-over-IP), and so on. With access to these records and by being able to modify them, you can have your site with company A and your e-mails with company B, for example. This gives you more independence as you can choose the best provider for every single one of these services and you don't need to stick to one company. As soon as you pick a hosting company, you have to make sure that you can access all DNS records freely because some companies have an additional fee for this function.