Why would I want to put information on my web page that is not visible to the typical viewer?

I would want to put information on my webpage that is not visible to the typical viewer because it may be necessary for the webpage to function properly but if someone just viewing the webpage sees it they would be confused since they won't know what it's for. For instance, the typical viewer might not be able to see the font, font size, the colors, or the metadatas in the webpage and that makes it simpler for to viewer to understand the page. If everything in the code was visible for the viewer they won't which is which and would get overwhelmed and find the webpage useless.