> If you only care about headers use the -I flag and the response body will be hidden
That is actually wrong. The -I flag set the request method to HEAD. So in some cases it will return different headers than a normal get request (and some servers don't implement HEAD responses at all).
(HTTP) Include the HTTP-header in the output. The HTTP-header includes things like server-name, date of the document, HTTP-version and more...
(HTTP/FTP/FILE) Fetch the HTTP-header only! HTTP-servers feature the command HEAD which this uses to get nothing but the header of a document. When used on an FTP or FILE file, curl displays the file size and last modification time only.