BFoB Case Study question--table summaries

In looking at the source code of the site itself
(http://www.birminghamfocus.org.uk), I see that all the tables have
summaries (for example, "This table is a spacer" and "This table contains
the main site navigation bar").

Is this necessary? I was under the impression that only data tables needed
summaries. Does providing a summary for tables used for layout serve a
purpose?

Thanks!
leslie