rethrowing exceptions

[rubing]

I have been reading that it is common to rethrow exceptions in catch blocks.  Why would one want to do this.  Wouldn't that just leave you with an unhandled exception.  For example:

 

$variable = NULL;

try
{
  $variable = some_kind_function();
  another_crazy_function($variable);
}
catch (Exception $e)
{
failure causes clean-up and rethrow
if ($variable != NULL)
  close_function($variable);
throw $e;  //WHY RETHROW???
}

//rest of script ...
close_function($variable);

 

[rubing]

Well, I forgot to mention that you can set up a custion exception handler for unhandled exceptions.  But still why would anyone want to re-throw an exception, instead of just handling it.  I just don't get it!