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The Host request header specifies the host and port number of the server to which the request is being sent. If no port is included, the default port for the service requested is implied (e.g., 443 for an HTTPS URL, and 80 for an HTTP URL). A Host header field must be sent in all HTTP/1.1 request messages.
Common Response Headers The first line of the response is mandatory and consists of the protocol ( HTTP/1.1),response code (200)and description (OK). The headers shown are: CONTENT-Type -This is Text/html which is a web page. It also includes the character set which is UTF-8.
The HTTP Host represents the domain name of the server. It may also represent the Transmission Control Protocol (TCP) port number which the server uses. Defining the port number is optional, the default value is considered.
If your question is "why specify the host in a Host header as opposed to on the Request-Line", the answer is the need for interopability between HTTP/1.0 and 1.1. If the question is "why is the Host header mandatory", this has to do with the desire to speed up the transition away from assigned IP addresses.
See section 14.23 of the HTTP spec which specifies that the port # should be included if it's not the default port (80 for HTTP, 443 for HTTPS).
UPDATED for modern day browsers:
Browsers (and curl) will add the port only when it is not the standard port, as required by the HTTP spec and noted in @superfell's answer.
Browsers this day (2013), will actually strip the port from the Host Header when the port is the standard (http port 80, https port 443). Some clients, which use their own method, like the Baidu Spider, include the port number even when the port is 80.
Whether this is proper or not, I don't know. The spec doesn't say whether it's OK or not to include the port number when the port used IS the default.
To answer your comment, servers will do whatever they need to do to comply with the spec, and the spec suggests only the cases WHEN it's needed. Because of this, I feel It's not really a question of how the server deals with it - it's more how the client issues the request: includes the port number in the Host Header, or not.
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