The term “hosting” doesn't describe a single service, but a number of services that provide different functions to a domain name. Having a website and emails, as an example, are two independent services although in the general case they come together, so many people see them as one single service. In reality, each and every domain has a couple of DNS records called A and MX, which show the server that manages each specific service - the former is a numeric IP address, that specifies where the site for the domain is loaded from, while the second one is an alphanumeric string, which shows the server that manages the emails for the domain name. As an illustration, an A record is 22.214.171.124 and an MX record can be mx1.domain.com. Each time you open a website or send an e-mail, the global DNS servers are contacted to check the name servers that a domain name has and the traffic/message is first directed to that company. When you have custom records on their end, the Internet browser request or the e-mail will then be forwarded to the correct server. The reasoning behind employing separate records is that the two services use different web protocols and you may have your site hosted by one provider and the e-mail messages by another.