Why Have Cells Selected Reactive Oxygen Species To Regulate Cell Signaling Events?

There is a growing body of evidence demonstrating that exposure of cells to reactive oxygen species (ROS) leads to oxidative modification of nucleic acids, proteins, and lipids, and that such modifications can contribute to the development of a number of diseases and aging. This raises the question: If ROS are so damaging to cells, why have cells selected ROS to trigger activation of so many cell signaling pathways?