What are these datepicker widget files for

What are these datepicker widget files for

I downloaded the datepicker widget (just the datepicker) from  this site and i got all of these file and folders:

However I got the datepicker working ok by just referring to two of them ( query-ui.css and  /jquery-ui.js) in the following:

  2. <!doctype html>
  3. <html lang="en">
  4. <head>
  5.   <meta charset="utf-8">
  6.   <meta name="viewport" content="width=device-width, initial-scale=1">
  7.   <title>jQuery UI Datepicker - Default functionality</title>
  8.   <link rel="stylesheet" href="jquery/jquery-ui.css">
  9.   <script src="jquery/jquery-3.2.1.min.js"></script>
  10.   <script src="jquery/jquery-ui.js"></script>
  11.   <script>
  12.   $( function() {
  13.     $( "#datepicker" ).datepicker(
  14.     {
  15.      changeMonth: true,
  16.      changeYear: true,
  17.      yearRange: "1949:2025"
  18.     }
  19.     );
  20.   } );
  21.   </script>
  22. </head>
  23. <body>
  24.  <p>Date: <input type="text" id="datepicker"></p>
  25.   
  26. </body>
  27. </html>
Where can I find out what these other files are for and when to I need to referred to them?