Any active domain which is added in a hosting account has a variety of records that enable 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 (using an Internet domain for non-website purposes on a specific port number like Voice-over-IP), etc. With access to these records and by being able to edit them, you could have your website with company A and your e-mail messages 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 only one company. When you choose a hosting company, you have to make sure that you have access to all DNS records freely since some providers have an extra fee for this feature.