Any active domain which is added in a hosting account features a number of records which allow it to function properly. In the standard case, the name servers (NS records) show the location where the domain is hosted and they are 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 a domain address for non-website purposes on a certain port number like Voice-over-IP), and many others. Having access to these records and by being in a position to modify them, you can have your site with company A and your e-mails with company B, for example. This offers you more independence since you can select the best provider for every one of these services and you don't need to stick to a single company. After you choose a hosting company, you have to make certain that you can access all DNS records freely because some companies have an extra fee for this function.