why we use keyup function in javascript code example

Example 1: javascript on keypu

<input type="text" onkeyup="myFunction()">

Example 2: jquery keyup

// Similar to KeyUp but just called on input change (optimal for AJAX calls)
  var input = $('#formID').find('input[type="text"]');
  input.on('input',
    function(){
      console.log (input.val());
    });