Any active domain which is added in a hosting account features a variety of records that allow it to function properly. In the typical case, the name servers (NS records) show where the Internet domain 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 like A (IP address of the website/server), MX (email server), SRV (employing a domain for non-website purposes on a specific port number like Voice-over-IP), etc. With access to these records and by being in a position to modify them, you may have your website with company A and your e-mails with company B, for example. This gives you more overall flexibility because you can pick the best provider for every single one of these services and you do not have to stick to one company. After 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.