With introduction of stacking segments in Analytics we're able to reduce the no. of segments we create as we're able to stack various combination. Unfortunately this feature is not available in DW and therefore we have to create nested segments specifically for DW.
We export our data for auditing purposes for around 15 sites. 'One' of the metrics is Single Page Visits thus we have to create 15 segments instead of using the segments 'site x' and 'single page visits'