In a turbocharged diesel engine, the exhaust system primarily serves to:

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The fundamental idea here is energy recovery from the exhaust to boost engine intake air. In a turbocharged diesel, the exhaust gas leaving the engine is hot and high-velocity. It passes through the turbine side of the turbocharger, and that flow spins the turbine wheel. The turbine is connected to the compressor on the intake side, so as the turbine turns, it drives the compressor to force more air into the engine. More air allows more fuel to be burned efficiently, increasing power and overall efficiency. That energy extraction to drive the turbocharger is the primary role of the exhaust system in this setup.

Cooling the exhaust gas isn’t the main objective; the exhaust remains hot for the turbine energy transfer and for after-treatment processes. Reducing engine noise is a function of the muffler, not the essential purpose of powering the turbocharger. Increasing backpressure isn’t the goal either; the system is designed to balance backpressure while capturing enough energy to spin the turbine.