Any active domain name that's added in a hosting account features a number of records that permit it to work properly. In the general case, the name servers (NS records) show where the domain name 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 (using a domain name for non-website purposes on a certain port number such as Voice-over-IP), and so on. Having access to these records and by being in a position to modify them, you may have your web site with company A and your e-mails with company B, for illustration. This offers you more independence as you can select the best provider for every one of these services and you don't have to stick with only one company. Whenever you select a hosting company, you have to make sure that you have access to all DNS records freely since some companies have an additional fee for this function.