Well, there are a number of good reasons, but looking through the list of enhancements is this one long overdue feature:
For languages that are written right to left, such as Hebrew and Arabic, Snow Leopard now elegantly handles mixing in left-to-right text. It also has a split-cursor option that shows the appropriate cursor direction at the boundary between right-to-left and left-to-right text.
You can read more about it here. Hopefully this means that inserting RTL text in the midst of LTR text also works, since there’s much room for improvement in the way Mac OS X handles that as well (Pages and Keynote simply do not handle putting Unicode Hebrew into your documets, fortunately we have Mellel to solve our problems).