What does threshold feel like? Some (rare – think Fabian Cancellara) athletes know, but most of us train lower than threshold, when we think we’re riding that hard. With a power meter (accurately calibrated and zeroed) you know what threshold is – the number is displayed in front of you.
Cyclists that train with power typically train much harder when they are training hard and much easier when they’re recovering.
Training with power means training at the right intensity to maximize the training effect of your given workout.