The host header in a http request provides the host(the domain) and port information from the target URI . For example, a GET request to the origin server for<http://www.example.org/pub/WWW/> would begin with:

     GET /pub/WWW/ HTTP/1.1
     Host: www.example.org

This is useful in case of

note that host is a forbidden header hence it cannot be programatically modified in browser. Read more here.https://developer.mozilla.org/en-US/docs/Glossary/Forbidden_header_name.  However host header can be modified via hacks. Readm more https://stackoverflow.com/questions/21382813/how-can-i-modify-the-host-header. Also proy like burp proxy and intercept requests and then view , modify all requests and response. (proxy do not have browser restrictions obviously)