Any active domain name that's added in a hosting account includes a number of records that enable it to function properly. In the general 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 specific records set through the hosting company like A (IP address of the website/server), MX (email server), SRV (employing an Internet domain for non-website purposes on a specific port number like Voice-over-IP), etc. By having access to these records and by having the capacity to modify them, you could have your web site with company A and your e-mails with company B, for instance. This gives you more independence because you can choose the best provider for every single one of these services and you don't need to stick with only one company. As soon as you select a hosting company, you have to make sure that you have access to all DNS records freely as some companies have an additional fee for this function.