Any active domain which is added in a hosting account includes a number of records that permit it to function 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 specific records created through the hosting company like A (IP address of the website/server), MX (email server), SRV (using an Internet domain for non-website purposes on a certain port number like Voice-over-IP), and so on. With 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 example. This gives you more independence because you can select the best provider for every single one of these services and you don't have to stick with one company. As soon as you pick a hosting company, you have to make sure that you can access all DNS records freely since some providers have an additional fee for this feature.