Any active domain name which is added in a hosting account features a variety of records that enable it to perform properly. In the typical case, the name servers (NS records) show the location 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 created 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 certain port number like Voice-over-IP), and many others. Having access to these records and by having the capacity to modify them, you can have your website with company A and your e-mails with company B, for example. This gives you more independence because you can pick the best provider for every one of these services and you don't need to stick to a single company. As soon as you pick a hosting company, you have to make sure that you can access all DNS records freely as some companies have an additional fee for this feature.