Any active domain name that's added in a hosting account has a variety of records that enable it to function properly. In the standard case, the name servers (NS records) show the location where the domain name 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 such as A (IP address of the website/server), MX (e-mail server), SRV (using an Internet domain for non-website purposes on a specific port number such as Voice-over-IP), etc. Having access to these records and by being in a position to modify them, you could have your site with company A and your e-mails with company B, for example. This gives you more flexibility since you can choose the best provider for every one of these services and you don't have to stick with one company. As soon as you choose a hosting company, you have to make sure that you have access to all DNS records freely because some providers have an additional fee for this feature.