Examining headers is sufficient for many checks and statistics. For example, we don't need to download an entire page to confirm it is reachable. Just reading the HTTP response is sufficient.

Another use case for examining the HTTP header is to check the Content-Length field to determine the file size or the Last-Modified field to see if the file is newer than a current copy before downloading.

The -I or -head option outputs only the HTTP headers, without downloading the remote file:

$ curl -I http://knopper.net 
HTTP/1.1 200 OK
Date: Tue, 08 Nov 2016 17:15:21 GMT
Server: Apache
Last-Modified: Wed, 26 Oct 2016 23:29:56 GMT
ETag: "1d3c8-1af3-b10500"
Accept-Ranges: bytes
Content-Length: 6899
Content-Type: text/html; charset=ISO-8859-1