Any active domain that's added in a hosting account features a number of records that enable it to work properly. In the common case, the name servers (NS records) show where the domain name is hosted and they are set on the side of the registrar company, but on the lower level there are certain records created through the hosting company like A (IP address of the website/server), MX (e-mail server), SRV (using a domain for non-website purposes on a certain port number like Voice-over-IP), etc. 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 example. This gives you more overall flexibility since you can choose the best provider for every one of these services and you do not have to stick with one company. When you select a hosting company, you have to make sure that you can access all DNS records freely as some companies have an extra fee for this feature.