what is the use of WebHeaderCollection while downloading a web site,explanation

Question

Headers = new WebHeaderCollection();
RequestUri = uri;
Headers["Host"] = uri.Host;
Headers["User-Agent"] = "Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.0.11) Gecko/2009060215 Firefox/3.0.11 (.NET CLR 3.5.30729)";
how will this command work........