Any active domain which is added in a hosting account features a number of records that allow it to perform properly. In the standard case, the name servers (NS records) show the location where the domain is hosted and they're set on the side of the registrar company, but on the lower level there are particular records set through the hosting company like A (IP address of the website/server), MX (e-mail server), SRV (employing a domain name for non-website purposes on a particular port number like Voice-over-IP), etc. By having access to these records and by having the capacity to edit them, you can have your web site with company A and your emails with company B, for illustration. This offers you more overall flexibility because you can select the best provider for each of these services and you don't need to stick to one company. As soon as you select a hosting company, you have to make sure that you have access to all DNS records freely since some companies have an extra fee for this feature.