This property affects only bar series with the type RMC_BARSINGLE. RMC_BARGROUP, RMC_BARSTACKED and RMC BARSTACKED100 have per definitonem more then one data point per column (RMC_BARGROUP side by side, the stacked bars on top of each other).
Normally you will define a label axis with as much columns as you have data points; in this case set PointsPerColumn to 1 (or omit it, so it defaults to 1). But imagine the following: you want to draw whatever for the first and the second half of a year on a monthly basis. You could add a label axis with two columns (and labels), add a bar series with type RMC_BARSINGLE, pass 12 data values and set PointsPerColumn to 6. In this case you have for each term six separated bar objects.