what does this symbol mean in this perticular case?

what does this symbol mean in this perticular case?

you can see  "-="  before  "+" symbol in this statemnt, i know
x -= y  means
x = x - y
but here why this symbol is used though if remove the symbol also iam getting the same result.. pls tell me why this ymbol is used?

  1. $(this).animate({'left':'-=' + scrollAmount}, 'slow');

actually this is a thumnile scroller code

  1. $('#photos_inner').toggle(function() {
  2. var scrollAmount = $(this).width() - $(this).parent().width();
  3. $(this).animate({'left':'-=' + scrollAmount}, 'slow');
  4. }, function() {
  5. $(this).animate({'left':'0'}, 'slow');
  6. });