Any active domain name which is added in a hosting account features a number of records that allow it to perform properly. In the general 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 such as A (IP address of the website/server), MX (email server), SRV (using a domain name for non-website purposes on a certain port number like Voice-over-IP), etc. With access to these records and by being in a position to edit them, you could have your web site with company A and your e-mails with company B, for illustration. This offers you more freedom as you can select the best provider for every one of these services and you don't have 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 providers have an additional fee for this function.