Pivot table do the sub totals based on ur dimension taken in ur chart.
Suppose in ur chart u have country, region, state..... u have to see country wise total, region wise total, n state wise total..... it will helpfull in that case,
Here those values will show in bottom of ur selected dimension. for suppose if u selected region n sub totals in bottom, it will show the sum values for every region.
Hope it will help u.