Why do requirements matter?
It might be easier to understand why you need requirements if you can see how a site will come to life in Drupal. Of course I can't tell you how to build a site in this short introduction but I can offer you the illustration and companion chart below. The illustration represents a very simplified version of a system process.
Data is entered into a database. Drupal assembles the data into node and possibly a block or two that will be shown in your page. Your pages come together to form your site structure connected via various forms of navigation.