Why is Diesel–Electric engine more effective than direct diesel engine?

Why do converting mechanical power to electric power then convert electric power back to mechanical power is more effective than directly use mechanical power?

They are most certainly not more efficient. Efficiency is not the reason that diesel-electric locomotives or monster mining trucks exist. The reason is that the electric motor can produce enormous torque at startup without needing a container-sized gearbox. The 2nd advantage is that power can be directly applied to all 10 or 12 locomotive wheels without needing any gearing or coupling rods.

Note the lack of a gearbox and the comparatively small diesel engine….