The jet engine is at its most efficient at about 90% RPM. At 230KTS at sea level the engine will be nowhere near its most efficient operating range. And if you try to get the engine up to 90% RPM then you will, in all probability over speed the aircraft.
So we have to take the aircraft to a level where the engine can operate at its most efficient, and where it is supplying enough thrust so as to keep the aircraft in the sky and not over speed it. This is done by reducing the mass airflow into the engine (climbing to high level) and thus the amount of fuel required to burn that mass air flow is subsequently reduced.
So the engine is not improving, the performance of it is getting worse as you climb. However it has far too much performance at sea level, so we need to reduce that performance to a level that means that we can operate it at its most efficient RPM without damaging the aircraft.
If that makes sense?