Concurrency is all about doing multiple things at the same time, and it probably matters now more than it ever has before. Consider a CPU running at 3 GHz, or 3x10^9 cycles per second. The speed of light in a vacuum is 3x10^8 meters/second, which matters for us because we cannot move information faster than this. Ah, and your CPU isn’t a vacuum so the speed that information moves around a CPU is slower than that. Even data did move at the speed of light, that still only allows for up to 10cm of data movement around a circuit per cycle.

This is not all that far, and you can see that scaling clock speeds up further and further will not be sustainable. Making the circuits smaller isn’t sustainable either – you must have sufficient space between “wires” in the chip or you get current leakage and capacitance effects (essentially, electrical problems). And let’s not even talk about quantum tunnelling (yes, we really are getting down to the sizes when quantum effects start to become a real issue)...