What's the difference between two set of codes, I know what arrow function does but why traditional expression does not work?

What's the difference between two set of codes, I know what arrow function does but why traditional expression does not work?

Set - 1

loop through all elements

    checkboxes.forEach( function (checkbox){
        console.log(checkbox);

        if( checkbox === this || checkbox === lastChecked)
        {
            inBetween = !inBetween;
            console.log(" start checking them inbetween");

        }

        if(inBetween)
        {
            checkbox.checked = true;
        }
    });

Set - 2

loop through all elements

  checkboxes.forEach(checkbox => {

  console.log(checkbox);
  if (checkbox === this || checkbox === lastChecked) {
    inBetween = !inBetween;
    console.log('Starting to check them in between!');
  }

  if (inBetween) {
    checkbox.checked = true;
  }
});