Lionheart,
When set to 100psi your pumps will be delivering something in the order of 80psi when the trigger valve is open (dependant on jetting)
In simple terms, try cleaning a concrete path with a garden hose, then a pressure washer.
That is not a true comparison but does indicate the power of a jet of water to dislodge soil. Being able to deliver water at a greater flow-rate also enables the same or greater flushing to be achieved on a shorter time-scale.
Whilst there are other factors to be taken into consideration, such as balancing the water put down to the recovery potential, that is the short answer.