Any active domain name that's added in a hosting account features a number of records that permit it to work properly. In the common case, the name servers (NS records) show the location where the Internet domain is hosted and they are set on the side of the registrar company, but on the lower level there are particular records created 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 certain port number such as Voice-over-IP), and many others. By having access to these records and by having the ability to modify them, you can have your website with company A and your e-mails with company B, for instance. This offers you more overall flexibility since you can pick the best provider for every one of these services and you don't need to stick with one company. Whenever you pick 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 function.