> I have understood - and used - it as a row that sums up the data in a  
> table that contains numerical information (such as financial reports)..

It isn't.  It's a running footer for each page of the table, like
thead is a running header.

Note that at least some versions of IE have a default style sheet that
does not render thead and tfoot as running headers.  I believe this
was a sort of quirks mode type decision, i.e. it was deliberately 
mis-implemented in order to avoid confusing authors.