Why do some diesel engines have throttle valves?

The air intake of diesel engine is fixed, and the power can be changed by controlling the injection of different amounts of diesel oil. If the throttle valve is added here, it will increase the intake resistance, cause intake loss and affect the working efficiency of the diesel engine. At present, there are also diesel engines equipped with throttle valves, mainly for the consideration of tail gas aftertreatment, to control the exhaust volume of recirculated EGR, improve the combustion environment and reduce NO compounds. I don't know if you said this throttle.