It depends on whether you are in a situation where demand outstrips supply (surge pricing) or supply outstrips demand (UberX). By lowering the price, they are

It depends on whether you are in a situation where demand outstrips supply (surge pricing) or supply outstrips demand (UberX). By lowering the price, they are trying to get more people to opt for UberX vs. taxi/Lyft/etc which theoretically brings the supply/demand into equilibrium. There is potentially the effect of lowering prices so much that drivers don't find it worthwhile to pick up fares in which case you once again help get into equilibrium since now supply is closer to demand levels. If Uber wants to counter that secondary effect, they can lower the prices by reducing their own margin w/o impacting the driver's cut.