Hi Ted,
Just a quick note:
1. They allow you to exactly position table cells so your page can layout
exactly as you want it to -- since it defaults to using specific table
sizes, you don't need to worry about the size of the browser (like you do
when you use percentages). You know that your table will always render at
some specific width. Alternatively, you can easily change them to expand to
100% of the browser as well as specify which column should expand with the
table (so if you have a three column table you can set it to 100% and then
set the middle column to be the one that expands as the browser expands).
2. Ignoring #1 above, though, my favorite feature is Rounded Corners. After
you insert a layout cell, select it (so you see the blue border) and show
the layout tables Taskpane, then click on Cell Formatting and add a corner
to your table, or maybe a shadow, or just a border.