Any active domain name that's added in a hosting account has a variety of records that allow it to function properly. In the typical case, the name servers (NS records) show the location where the Internet domain is hosted and they're set on the side of the registrar company, but on the lower level there are certain records set through the hosting company like A (IP address of the website/server), MX (e mail server), SRV (employing a domain address for non-website purposes on a certain port number like Voice-over-IP), etc. Having access to these records and by being in a position to edit them, you can have your website with company A and your e-mails with company B, for illustration. This offers you more independence since you can pick the best provider for each of these services and you don't need to stick with a single company. As soon as you pick a hosting company, you have to make sure that you can access all DNS records freely as some companies have an additional fee for this function.