Originally Posted by FBinNY

Even top end road bikes are moving toward some degree of sloping top tube. Modern seatposts with higher strength make the sloping top tube design practical. The main benefit is lower standover height, for the same handlebar height. This is especially valuable with the change to threadless headsets, which reduce the ability to raise the bar with respect to the top of the headset.

So, you can think of a sloping top tube as lowering the standover height or raising the headtube/handlebar height, for the same rider and saddle height.