Any active domain name which is added in a hosting account features a number of records that permit it to perform properly. In the common 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 (e-mail server), SRV (using an Internet domain for non-website purposes on a particular port number like Voice-over-IP), etc. With access to these records and by being able to modify them, you could have your web site with company A and your emails with company B, for illustration. This offers you more independence as you can pick the best provider for each of these services and you don't need to stick to only one company. Whenever you select a hosting company, you have to make certain that you can access all DNS records freely because some providers have an extra fee for this feature.