I would want to put information on my web page that isn't visible to the typical viewer for the program to understand what information I'm putting on my site. For example, if I want to put an image on my site, I'll add the code to put the image along with the image adress for it to show up on my website. I also use it to embellish and refine the information that is already visible, such as background colors. It would make the page more visibly appealing and easy to look at.
Another thing we need to use that the viewer doesn't see is the meta data. This organizes all the data and actually helps the author see all the attributes of the page up top, like the language and the description. The viewport additionally controls how the page fits on the screen for the comfort of the viewer.