To add to what Jerry said, buffer capacity can be looked at as the total number of moles of alkali present in a solution. Thus, water has zero buffer capacity and lets say that it is at pH 7 (protected from CO2 in the air of course). So, if we add a pH 7 buffer, then the buffer capacity rises but the pH stays constant.

Addition of dilute acid or base to plain DW will change the pH rapidly, but adding dilute acid or base to the buffered DW will cause a slower change if any takes place at all.