Why are multiple aspect signalling systems used? What was wrong with the older systems which had two aspects?

Multiple aspect signals, by providing several intermediate speed stages between 'clear' and 'on', allow high-speed trains sufficient time to brake safely if required. This becomes very important as train speeds rise. Without multiple-aspect signals, the stop signals have to be placed very far apart to allow sufficient braking distance, and this reduces track utilization. At the same time, slower trains can also be run closer together on track with multiple aspect signals.

Source - IFRCA.org