When studying transonic flight it is tempting to think that something very dramatic occurs at Mcrit, but this is not the case. Mcrit is simply the lowest mach number at which any part of the airflow over the surfaces of an aircraft reaches the speed of sound. Because this airflow is only just sonic it produces few if any shock waves. And because the sonic airflow is restricted to a very small region of the aircraft surface, any shock waves formed affect only a very minor part of the overall flow. Because of these factors, there is no significant increase in drag, and no significant changes in aircraft performance or handling at Mcrit. So Mcrit is really just of academic interest to pilots.
why would you want to fly at speeds that are going to increase your drag?
In order to achieve maximum endurance we must fly at the speed at which fuel consumption rate is lowest. For jets this is generally accepted to at or about Vmd. In order to achieve maximum range for a given fuel low we must fly at the speed at which we achieve the maximum ratio of TAS : Fuel Flow. In jet aircraft fuel flow is proportional to thrust and in steady flight thrust is equal to drag. So for minimum fuel consumption between two points (and maximum range) we need to fly at the speed at which we achieve the maximum TAS : Drag ratio.
If we look at a graph of drag against TAS for a typical jet aircraft at subsonic speeds, the curve is a shallow bucket shape with the minimum drag value at Vmd. As speed increases beyond Vmd the drag increases, but the rate of increase is initially very small. This means that at speeds slightly greater than Vmd, the rate of increase in drag is less than the rate of increase in TAS. This means that increasing the TAS within this speed range increases the drag slightly, but actually increases the TAS : Drag ratio. But if we continue to increase our TAS, the drag curve becomes steeper, and a speed is eventually reached where the drag is increasing faster than the increasing TAS.
The Drag Divergence Mach Number (Mdd) is a mach number slightly higher than Mcrit, at which the shock waves become sufficiently widespread and intense to create a large increase in drag. This causes a marked decrease in the TAS : Drag ratio. So at speeds between Mcrit and Mdd increasing airspeed will have a beneficial effect on the TAS : Drag ratio.
To find the speed at which we get the best TAS : Drag ratio we draw a tangent from the origin of the graph to touch the drag curve. The speed at which this line touches the drag curve is called Velocity Maximum Cruise Range (Vmrc). Vmrc gives the minimum fuel consumption between two points, which in turn means that it gives the maximum attainable range for any given fuel load.
But as Genghis has said, the fuel used does not represent the full cost of flying from A to B. The full cost includes Crew salaries, aircraft depreciation/lease costs, ATC charges and many others. Some of these costs are time-related and some are not. Flying faster than Vmrc increases some of these costs and decreases others. So the lowest overall cost is achieved by flying at the speed which gives the best compromise between these different costs. This speed is usually slightly faster than Vmrc, which itself is faster than Mcrit.